Dynamics of Retail Price Improvement Orders: Market Microstructure, Institutional Interplay, and Regulatory Evolution

Introduction to Market Microstructure and Order Segmentation

The architecture of modern equity and options markets is defined by the perpetual struggle to balance liquidity provision against the risks of adverse selection. Within the national market system, order flow is not a homogenous entity; it is fundamentally bifurcated into institutional and retail classifications. Institutional order flow is typically executed in massive blocks, algorithmically sliced over hours or days, and often carries high informational value. When a market maker or institutional liquidity provider interacts with this informed flow, they face the severe risk of adverse selection—the probability that the market price will move against their position immediately following the execution. Consequently, liquidity providers demand a wider bid-ask spread to compensate for the toxic nature of institutional flow.

Conversely, retail order flow is overwhelmingly characterized as uninformed. Retail investors generally trade in smaller sizes, react to publicly available information rather than proprietary latency-arbitrage signals, and do not possess the directional toxicity that market makers fear. Because the risk of assuming the other side of a retail trade is statistically minimal, liquidity providers are willing to offer better prices—known as price improvement—specifically to capture this benign flow.

The mechanism through which this price improvement is delivered, and the venue upon which the execution occurs, represents the central battleground of contemporary market microstructure. Retail Price Improvement (RPI) orders and their associated Retail Liquidity Programs (RLPs) were engineered by public lit exchanges, such as the New York Stock Exchange (NYSE), Cboe Global Markets, and Nasdaq, as a competitive response to off-exchange internalization. For years, wholesale market makers have captured the vast majority of retail flow off-exchange by paying retail brokerages for the exclusive right to execute their clients’ orders. By deploying RPI programs, public exchanges created a highly specialized, sub-penny execution framework that allows institutional market makers to post hidden liquidity dedicated exclusively to interacting with incoming retail orders.

This comprehensive report exhaustively details the technical mechanics of Retail Price Improvement orders, the economic equilibrium that balances retail takers against institutional providers, the transformative impact of the Securities and Exchange Commission’s (SEC) Regulation NMS amendments regarding minimum pricing increments, and comparative international microstructure frameworks that highlight alternative approaches to market segmentation.

The Structural and Technical Architecture of Retail Price Improvement Orders

To bridge the substantial gap between transparent lit exchanges and the off-exchange dark pools utilized by wholesalers, exchanges developed specific order protocols that permit market makers to passively capture marketable retail demand throughout the trading day Cboe RPI. The RPI framework relies on a rigid categorization of market participants, strict order routing instructions, and the utilization of the Financial Information eXchange (FIX) and Binary Order Entry (BOE) protocols.

Market Participant Typology: RMOs and RLPs

The architecture of Retail Liquidity Programs strictly segregates market participants into two distinct categories to ensure that institutional flow cannot masquerade as retail flow to capture sub-penny prices.

The first category consists of Retail Member Organizations (RMOs). These are exchange members or broker-dealers that have undergone a rigorous approval process by the exchange to submit Retail Orders (ROs) Nasdaq BX RPI. To qualify as an RMO, a member must conduct a retail business or handle retail orders on behalf of another broker-dealer, complete a specific application, and designate certain order entry ports exclusively as “Retail Only Ports” Nasdaq BX RPI. Crucially, the orders routed through these ports must be agency or riskless-principal orders originating from a natural person; they absolutely cannot be generated by a trading algorithm or a computerized methodology Nasdaq BX RPI. RMOs act as the vital conduit for retail flow, routing these orders to the exchange to seek the highest possible price improvement for the retail investor.

The second category encompasses Retail Liquidity Providers (RLPs). These are institutional market makers, proprietary trading firms, and liquidity adders who submit RPI orders to the exchange Cboe RPI. These market participants are willing to offer price improvement over the National Best Bid or Offer (NBBO) in minimum increments of $\$0.001$ specifically because they are guaranteed that they will only interact with the benign flow originating from RMOs Cboe RPI. Because RPI orders do not interact with resting retail orders or other institutional liquidity-seeking orders, the institutional providers are thoroughly insulated from adverse selection Cboe RPI. This structural protection allows them to quote much more aggressively than they would on the public, displayed order book.

Network Protocol and FIX Order Mechanics

The successful implementation of RPI orders relies on the highly precise utilization of FIX protocol tags to ensure that the liquidity is correctly flagged, routed, and hidden from the broader market. Across prominent exchanges such as Cboe EDGX and BYX, RPI orders are submitted with explicit parameters that dictate their behavior within the central limit order book Cboe RPI.

To post an RPI order, the liquidity provider must configure several specific FIX instructions. The Routing Instruction, denoted by FIX Tag 9303, must be established as B for Book Only or P for Post Only Cboe RPI. This ensures the order rests on the exchange’s book rather than routing outward to away destinations. The Extended Execution Instruction, represented by FIX Tag 9416, must be set to T to explicitly designate the submission as a Retail Price Improving order Cboe RPI. Furthermore, the Display Indicator, located at FIX Tag 9479, must be set to I to render the order Invisible Cboe RPI. While the invisible status is implied by the RPI designation, any other value entered for the display indicator will result in an immediate rejection by the matching engine Cboe RPI.

Although the specific limit price and size of the resting RPI order remain completely hidden from the public tape, the exchange must still signal to the market that price-improving liquidity is available to attract RMO order flow. When an RPI order is successfully posted at a price better than the same-side NBBO, the exchange disseminates a Retail Liquidity Identifier (RLI) across its proprietary market data feeds and to the public Securities Information Processor (SIP) NYSE rule filing. This indicator informs the market that sub-penny retail liquidity exists in a particular symbol, prompting smart order routers to direct retail flow to that venue.

RPI orders carry equal priority with all other non-displayed liquidity on the exchange book Cboe EDGX notice. However, they are subject to strict execution rules; they will not execute at prices equal to the same-side NBBO because a minimum price improvement of at least $\$0.001$ is a strict systemic requirement Cboe EDGX notice. Furthermore, RPI orders are rendered ineligible for execution during locked or crossed market conditions, pausing their functionality until normal market operations resume Cboe EDGX notice.

Pegged Order Support and Dynamic Pricing

To manage inventory and respond to rapidly changing market conditions without submitting thousands of order modifications per second, institutional liquidity providers frequently utilize pegged RPI orders. Exchanges permit RPI orders to be entered with dynamic instructions that tether the order’s limit price to moving reference points, specifically the primary NBBO or the midpoint of the NBBO.

When submitting a Primary Peg RPI order, the institution sets the Execution Instruction (FIX Tag 18) to R Cboe EDGX notice. This requires an additional offset instruction, defined by the Peg Difference (FIX Tag 211), which allows the algorithm to support aggressive offsets in increments of one-tenth of a cent ($\$0.001$) Cboe EDGX notice. The resulting Primary Peg price is continuously calculated by taking the same-side NBBO reference price and adding or subtracting the defined offset Cboe EDGX notice.

Alternatively, institutions seeking to offer maximum price improvement can submit a Midpoint Peg RPI order by setting FIX Tag 18 to M for a standard Midpoint Peg, or m for a Midpoint Peg that will not trade in a locked market Cboe EDGX notice. These pegged architectures allow institutional algorithms to maintain passive, price-improving quotes that drift organically with the broader market, reducing message traffic and computational overhead while maximizing the probability of intercepting retail flow.

The Typology and Handling of Retail Orders

When an RMO submits a Retail Order to interact with this resting RPI liquidity, the order is subjected to its own set of stringent technical constraints. Most notably, Retail Orders are strictly limited to Immediate or Cancel (IOC) Time-in-Force (TIF) instructions Nasdaq BX RPI. If an RMO erroneously submits a Retail Order with a TIF longer than an IOC—such as a Day order or Good-Till-Canceled—the exchange’s matching engine will automatically convert the order into an IOC to prevent retail flow from resting passively on the book Nasdaq BX RPI. Additionally, these orders must be submitted with an explicit limit price and cannot be submitted in sub-penny increments themselves; only the resting RPI orders may utilize sub-penny pricing Nasdaq BX RPI.

Retail orders are generally classified into distinct interaction paradigms based on the RMO’s desired routing behavior. A Type 1 Retail Order, often designated as “Price Improvement Only,” is configured to interact exclusively with resting price-improving RPI liquidity Nasdaq BX RPI. To execute this, the RMO sets FIX Tag 9416 to P, Tag 59 to 3 for IOC, and Tag 9303 to B for Book Only Cboe RPI. Under a Type 1 instruction, any shares that remain unexecuted after interacting with the available RPI liquidity are immediately canceled back to the broker Nasdaq BX RPI. This order type guarantees that the retail investor will only execute at prices strictly better than the prevailing NBBO Nasdaq BX RPI.

Conversely, a Type 2 Retail Order allows for broader market interaction. Under this designation, the order first attempts to execute against the available sub-penny retail price-improving liquidity Nasdaq BX RPI. If shares remain unexecuted, the order is permitted to interact with standard, non-price-improving liquidity resting on the lit order book Nasdaq BX RPI. Furthermore, Type 2 orders may carry specific routing instructions that permit the remaining shares to execute at away destinations if the primary exchange is depleted of liquidity Nasdaq BX RPI.

An illustrative example clarifies the execution mechanics. Suppose the NBBO for a security is $\$10.00$ on the bid and $\$10.05$ on the offer, resulting in a midpoint of $\$10.025$. The exchange book contains a standard displayed limit order to buy 100 shares at $\$10.00$. Concurrently, an institutional liquidity provider submits an RPI order to buy 100 shares at $\$10.001$, which is posted to the book as non-displayed Cboe RPI. An RMO then submits a Retail Order to sell 100 shares at $\$10.00$. The matching engine evaluates the book and executes the Retail Order against the RPI order at $\$10.001$ Cboe RPI. The RPI order achieves price priority over the displayed limit order because it is ranked higher ($\$10.001 > \$10.00$), and the retail investor receives exactly $\$0.001$ in price improvement per share Cboe RPI.

Enhanced RPI and the Programmable Step-Up Logic

To further optimize the execution fill rates for institutional liquidity providers while simultaneously enhancing the magnitude of price improvement delivered to retail investors, exchanges have innovated beyond the base RPI framework. Slated for implementation pending regulatory relief and SEC no-action letters, the “Enhanced RPI” order type represents a significant evolution in programmable liquidity provision Cboe Enhanced RPI notice.

The Enhanced RPI architecture allows institutional liquidity providers to post a limit order with a user-defined “Step-Up Range.” This functionality is governed by a new FIX protocol field, Tag 25025, known as the StepUpAmount Cboe FIX specification. Specified in minimum increments of $\$0.001$, and ranging from a default of $\$0.000$ up to an aggressive maximum of $\$99.999$, the step-up amount acts as a conditional aggression parameter Cboe Enhanced RPI notice.

Under standard operating conditions, an Enhanced RPI order remains ranked at its base limit price on the exchange’s order book Cboe Enhanced RPI notice. If an incoming Retail Order is marketable at that ranked price, and there are no competing orders with higher price-time priority, the Enhanced RPI order will passively execute exactly at its ranked price Cboe Enhanced RPI notice.

However, the step-up logic is activated when the incoming Retail Order threatens to execute against a competing, better-priced limit order resting on the book Cboe RPI. If the institution’s algorithm determines that the incoming retail flow is highly desirable, the Enhanced RPI order will dynamically utilize its StepUpAmount to increase its execution price, up to the maximum limit of the defined range, specifically to gain queue priority over the competing order Cboe RPI.

For buy orders, the step-up amount is implicitly added to the bid price, whereas for sell orders, it is subtracted from the offer price Cboe Enhanced RPI notice. For example, if an institution places a Primary Peg Enhanced RPI order on the bid with a specified step-up range, the algorithm will dynamically add the step-up amount to the current ranked price of the pegged order whenever necessary to step ahead of higher-priority liquidity to secure the retail fill Cboe Enhanced RPI notice. This programmable aggression ensures that institutions can systematically capture retail flow without the necessity of continuously quoting at the absolute most aggressive sub-penny increment. They only pay the higher price (conceding more spread) when market competition dictates it is mathematically required to win the execution.

The Microeconomic Balance: Retail Takers vs. Institutional Providers

To fully understand how the price balances between the retail investor and the institutional market maker, one must examine the fundamental microeconomics of liquidity provision. Institutions generating revenue through bid-ask spread capture operate in a precarious environment. Their primary adversary is adverse selection. When a market maker trades against an informed institutional counterparty—such as a quantitative hedge fund executing a directional momentum strategy—the market price typically moves against the market maker immediately after the trade, generating a loss on the inventory acquired.

Retail order flow is highly prized because it lacks this informational advantage. Retail traders, executing small lots based on lagging public information or sentiment, do not precipitate immediate, toxic price dislocations. Consequently, a market maker can safely assume the other side of a retail trade, capture the spread, and manage their resulting inventory risk through sophisticated hedging without the constant, looming threat of a massive institutional wave running over their position PFOF and internalization overview.

Internalization versus Exchange RPI

The fierce competition to interact with this benign, profitable retail flow has fractured the equity markets, creating two distinct and fiercely competitive execution ecosystems: off-exchange internalization and on-exchange Retail Liquidity Programs.

The dominant paradigm in the United States is off-exchange internalization, driven heavily by Payment for Order Flow (PFOF). Retail brokerages, which largely rely on zero-commission business models to attract customers, route their clients’ marketable orders directly to massive wholesale market makers PFOF and internalization overview. These wholesalers compensate the broker with a PFOF fee in exchange for the exclusive right to interact with that order flow CFA Institute.

Upon receiving the order, the wholesaler internalizes the trade, matching it directly out of their own proprietary inventory rather than sending it to a public exchange PFOF and internalization overview. To satisfy their regulatory Best Execution obligations, the wholesaler offers a fraction of a cent in price improvement over the public NBBO PFOF and internalization overview. The wholesaler’s profit is derived from spread capture, inventory management, and adverse selection control, while the PFOF payment serves as their customer acquisition cost PFOF and internalization overview. Because the wholesaler possesses an absolute information advantage regarding the retail nature of the flow, they can comfortably trade against orders that are on the wrong side of short-term market momentum CFA Institute.

However, this internalization ecosystem severely harms public lit exchanges. Because wholesalers siphon the vast majority of marketable retail orders off-exchange into dark internalization pools, public exchanges suffer from a severe lack of retail participation CFA Institute. This leaves their public order books filled almost entirely with toxic, institutional-to-institutional flow, which forces market makers to widen their displayed quotes to protect themselves, degrading overall market quality.

Exchange RPI programs are the direct countermeasure to this phenomenon. By allowing institutional liquidity providers on the exchange to offer sub-penny price improvement exclusively to retail orders, the exchanges attempt to incentivize retail brokers to route their flow to the lit market rather than selling it to wholesalers Cboe RPI. The RPI framework democratizes the internalization process, allowing any registered institutional liquidity provider on the exchange to compete for retail flow on a level playing field, rather than restricting access only to the massive wholesalers who possess bilateral PFOF agreements with retail brokers Proof Trading.

The Dynamic Price Equilibrium

The ultimate price balance struck between the retail taker and the institutional provider is determined by the intersection of institutional liquidity demand and the competitive pressures of the sub-penny market. Extensive academic research indicates that the magnitude of retail internalization fluctuates inversely with institutional order flow University of Warwick study. When institutional demand for liquidity is high across the broader market, wholesalers actively internalize more retail order flow to offset their massive institutional positions, resulting in complex, U-shaped intraday return patterns and a positive relation between distant future returns and the magnitude of retail trade imbalances University of Warwick study.

In the RPI ecosystem, the institution acts as a highly calculating, passive provider. The institution utilizes quantitative models to determine the absolute maximum sub-penny price improvement they can offer while remaining net profitable after accounting for clearing costs, exchange fees, and inventory holding risks. The retail investor benefits directly from this algorithmic competition, consistently receiving a better execution price than the prevailing public quote.

For instance, if a dominant off-exchange wholesaler is offering $\$0.001$ of price improvement, an institutional algorithm operating on Cboe EDGX utilizing an Enhanced RPI order may set a StepUpAmount of $\$0.002$ to ensure they intercept the trade on the lit book Cboe RPI. The resulting market equilibrium is one where the retail investor achieves maximum price improvement driven by institutional bidding wars, while the institution secures risk-reduced inventory at a price slightly inside the lit spread.

Empirical Evidence of RPI Efficacy

To assess the true market impact and economic validity of RPI orders, one must examine the empirical data regarding execution quality, spread capture, and the continuous availability of price-improving liquidity. The NYSE Group’s Retail Liquidity Programs offer a clear, quantitative view into this dynamic.

During periods of extreme market volatility—such as those experienced during retail-driven meme stock trading frenzies—off-exchange volume tends to decrease slightly relative to historical averages, pushing more retail flow to lit venues equipped with RPI capabilities NYSE data analysis. In heavily traded securities, RPI liquidity is maintained almost continuously by institutional providers. For instance, in the highly active Nasdaq-listed SQQQ (ProShares UltraPro Short QQQ) Exchange Traded Product, NYSE RLP programs maintained retail price improvement liquidity nearly $100\%$ of the time, with liquidity resting simultaneously inside the quote on both the buy and sell sides for over $64\%$ of the trading day NYSE data analysis.

Table 1: Time With Retail Price Improvement Liquidity (Sample High-Volume Equities)

Symbol	Time on Bid Only	Time on Offer Only	Time Both Sides	Total RPI Uptime
SQQQ	$18.7\%$	$16.7\%$	$64.3\%$	$99.7\%$
GE	$19.4\%$	$22.4\%$	$55.7\%$	$97.6\%$
UVXY	$30.7\%$	$28.2\%$	$64.3\%$	$30.5\%$ (Non-cumulative)*
NOK	$36.6\%$	$20.7\%$	$8.7\%$	$66.0\%$
AAL	$17.0\%$	$29.7\%$	$9.3\%$	$56.0\%$
AMC	$15.7\%$	$13.4\%$	$8.0\%$	$37.0\%$

*Data aggregation notes from the source reflect specific time-weighting methodologies utilized by NYSE NYSE data analysis.

When compared to competing market microstructure models, RLP demonstrates a clear superiority in maintaining tight markets. Inverted venues (maker-taker inverted exchanges, which charge liquidity providers and pay rebates to takers) possessed lit liquidity at both the National Best Bid and Offer on average just $7.9\%$ of the time NYSE data analysis. In stark contrast, the NYSE RLP framework maintained an order inside the quote on one or both sides for an average of $21.3\%$ of the time, proving the institutional willingness to supply liquidity when adverse selection risks are mitigated NYSE data analysis.

Table 2: RLP Execution Summary Statistics

Metric	Top 25 Most Active Securities	All Other Securities
Effective/Quoted (EQv) Ratio	0.66	0.74
Average Order Size (shares)	686	558
% of Orders $\le 500$ shares	$82.3\%$	$83.4\%$
Effective Spread (bps)	8.84	8.55
Price Improvement (bps)	3.00	1.66
% Volume Executed vs. RPI	$81.46\%$	$90.04\%$

The data contained in Table 2 confirms that the price balance strongly favors the retail taker in terms of execution quality. The volume-weighted average price improvement for the top 25 most active retail securities was a substantial three basis points, reaching as high as 26.5 basis points in extreme volatility scenarios involving wide spreads NYSE data analysis.

Perhaps the most critical metric is the Effective/Quoted (EQv) ratio, which serves as a premier institutional metric of execution quality. An EQv ratio of 1.0 indicates that a trade occurred exactly at the quoted spread, while a number less than 1.0 indicates execution inside the quoted spread (price improvement). The RLP program achieved an impressive 0.66 EQv ratio for top securities, and 0.74 for all others, unequivocally proving that institutional providers utilizing RPI orders are successfully delivering off-exchange-equivalent execution quality on a highly regulated, public order book NYSE data analysis.

Regulatory Evolution: The Sub-Penny Rule and Exemptive Relief

The operational existence of RPI programs is not a natural byproduct of free-market mechanics; it relies entirely on a complex history of regulatory exemptions granted against established market structure laws, specifically Rule 612 of Regulation NMS, colloquially known as the “Sub-Penny Rule.”

The Origins and Rationale of Regulation NMS Rule 612

Implemented by the SEC in 2005 during the initial rollout of Regulation NMS, Rule 612 explicitly prohibits national securities exchanges, national securities associations, alternative trading systems (ATS), vendors, and broker-dealers from displaying, ranking, or accepting bids, offers, or indications of interest priced equal to or greater than $\$1.00$ per share in increments smaller than $\$0.01$ Federal Register. For stocks priced below the $\$1.00$ threshold, the minimum increment is constrained to $\$0.0001$ Federal Register.

The SEC’s original, primary rationale for establishing Rule 612 was to protect the integrity of the limit order book by preventing market participants from “jumping the queue” Traders Magazine. Without a minimum increment, a liquidity provider who had established price-time priority by posting a massive limit order at $\$10.00$ could have their priority usurped by a high-frequency trading firm stepping ahead with a quote at $\$10.0001$. The SEC determined that allowing participants to gain priority by offering an economically trivial better price would actively discourage institutions from posting deep limit orders, thereby degrading overall market transparency and liquidity depth Traders Magazine.

The Decade-Long Pilot Phase and Exemptive Relief

While Rule 612 successfully protected limit order priority, it inadvertently created a regulatory moat that protected off-exchange wholesalers. Because wholesalers execute trades off-exchange, they are permitted to offer sub-penny price improvement to retail orders to satisfy Best Execution requirements. Public exchanges, however, were strictly barred from accepting or ranking sub-penny orders due to Rule 612, rendering them entirely incapable of competing for retail order flow.

Recognizing this anti-competitive imbalance, exchanges like the NYSE, Cboe, and Nasdaq petitioned the SEC for exemptive relief under Rule 612(c), which grants the Commission the authority to bypass the Sub-Penny Rule if doing so is deemed necessary, appropriate in the public interest, and consistent with the protection of investors Federal Register.

The SEC concurred that segmenting retail flow and allowing lit exchanges to offer sub-penny price improvement ultimately benefited the retail investor by fostering competition. Consequently, beginning in 2012, the SEC granted limited, temporary exemptions allowing the NYSE and NYSE Amex to operate RLP programs on a pilot basis, permitting them to accept and rank orders in $\$0.001$ increments solely for interactions with retail flow exemptive-relief request.

This pilot phase required near-constant regulatory maintenance. Between 2012 and 2019, the exchanges submitted dozens of extension requests to prevent the pilot programs from expiring Federal Register extension order. The SEC continually granted these six-month or one-year extensions, citing the need for additional time to analyze the empirical data concerning the programs’ impact on market quality Federal Register extension order. Finally, after seven years of continuous operation under temporary exemptive relief, the Commission approved the NYSE Retail Liquidity Program to operate on a permanent basis in 2019, cementing the sub-penny RPI framework into the permanent architecture of the national market system NYSE rule filing.

The 2024-2026 Regulation NMS Overhaul: Tick Sizes and Access Fees

Despite the success of RLP programs, the broader equity market continued to suffer from structural inefficiencies. As trade commissions plummeted to zero and algorithmic efficiency skyrocketed, it became apparent that nearly a thousand NMS stocks—accounting for approximately $47\%$ of all U.S. trading volume and $28\%$ of all trades—had become “tick constrained” exemptive-relief request.

A tick-constrained stock is a highly liquid security that naturally desires to trade at a bid-ask spread much narrower than the mandated $\$0.01$ increment. Because Rule 612 artificially prohibited the spread from compressing below a penny, these stocks suffered from artificially wide quotes, massively elongated order queues, and unnecessarily high transaction costs for investors exemptive-relief request. The one-cent spread was no longer a reflection of supply and demand, but rather an outdated regulatory constraint exemptive-relief request.

In response, the SEC finalized monumental amendments to Regulation NMS in September 2024, representing the most significant overhaul to market microstructure in two decades exemptive-relief request. These amendments drastically alter Rule 612 (Minimum Pricing Increments) and Rule 610 (Access Fees), with full compliance mandated by November 2025 Sidley Austin. These changes possess profound implications for the delicate balance of power between retail flow, off-exchange wholesalers, and exchange RPI programs.

The Half-Penny Tick Regime

The SEC abandoned its initial, highly complex proposal for four distinct tick tiers and instead established a streamlined dual-tick-size regime for stocks priced equal to or greater than $\$1.00$ per share Davis Polk. The assignment of a stock’s minimum tick size relies entirely on a backward-looking quantitative metric known as the Time Weighted Average Quoted Spread (TWAQS) during a predetermined Evaluation Period Sidley Austin.

Under the new rules, the primary listing exchange for an NMS stock is responsible for calculating the TWAQS semi-annually over a three-month Evaluation Period Davis Polk. The first Evaluation Period runs from January through March, determining the operative tick size for May through October Sidley Austin. The second Evaluation Period runs from July through September, dictating the tick size for November through April of the following year Sidley Austin.

Table 3: Revised Minimum Pricing Increments (Rule 612 Amendments)

Price Per Share	Time-Weighted Avg. Quoted Spread (TWAQS)	Assigned Minimum Tick Size
$\ge \$1.00$	$\text{TWAQS} \le \$0.015$	$\$0.005$ (Half-Penny)
$\ge \$1.00$	$\text{TWAQS} > \$0.015$	$\$0.01$ (Penny)
$< \$1.00$	N/A	$\$0.0001$

(Source: SEC Rule 612 amendments summary, September 2024)

The SEC estimates that based on historical liquidity profiles, an overwhelming $74.3\%$ of stocks will fall into the $\$0.015$ or less TWAQS threshold, thereby migrating to the new $\$0.005$ (half-penny) increment tier Sidley Austin. By allowing the broader market to quote in half-pennies, the SEC essentially narrows the artificial spread that off-exchange wholesalers have historically relied upon to generate their massive profit margins. This reduction in the baseline minimum increment severely compresses the space available for RPI orders to offer sub-penny price improvement without stepping into outright sub-half-penny territory.

Access Fee Compression and Maker-Taker Disruption

Concurrent with the tick size reduction, the SEC recognized that existing exchange access fees would cause severe market distortions under a half-penny regime. Under the original Regulation NMS, Rule 610 capped the fee an exchange could charge to access its protected quotes at 30 mils ($\$0.003$ per share) for stocks over a dollar Sidley Austin. If the minimum tick size was reduced to $\$0.005$, a 30-mil access fee would consume a staggering $60\%$ of the entire minimum spread, totally skewing the economics of order routing and liquidity provision Davis Polk.

To align with the final revisions to Rule 612, the SEC amended Rule 610 to drastically lower the exchange access fee caps Davis Polk. For stocks priced at or greater than $\$1.00$, the access fee cap is reduced to a maximum of 10 mils ($\$0.001$ per share) Davis Polk. For stocks priced below $\$1.00$, the cap is reduced from $0.3\%$ of the quotation price to $0.1\%$ Davis Polk.

This fee compression represents a direct assault on the traditional maker-taker pricing model favored by major exchanges. Because exchanges use the access fees collected from liquidity takers to fund the rebates paid to institutional liquidity makers, the reduction of the fee cap from 30 mils to 10 mils mathematically forces a corresponding collapse in rebate levels Davis Polk. This dynamic shifts the economic balance of power, making it cheaper for retail and institutional takers to access lit liquidity, while simultaneously reducing the financial incentive for high-frequency trading firms to post passive liquidity strictly to harvest rebates.

Acceleration of Odd-Lot Transparency

Finally, the 2024 amendments accelerate the implementation of rules designed to inject unprecedented transparency into the market regarding smaller orders. The SEC modified the implementation schedule for the definitions of “round lot” and “odd-lot information,” requiring the inclusion of this data into the consolidated SIP market feeds by May 2026 Sidley Austin. Crucially, the amendments require the explicit identification and dissemination of the “best odd-lot order” (BOLO) Sidley Austin.

Because retail investors frequently trade in odd lots (orders of less than 100 shares), revealing the BOLO enhances order price transparency and provides the broader market with highly precise data points regarding retail sentiment and positioning SEC press release. This democratizes information that was previously utilized almost exclusively by sophisticated quantitative models via proprietary exchange feeds, further leveling the playing field between the retail taker and the institutional provider WilmerHale.

Options Market Divergence: Exchange-Sponsored PFOF

While the equity market battles over fractions of a cent via tick size adjustments and wholesaler internalization, the listed equity options market operates under a profoundly different structural paradigm. Due to strict clearing considerations and the regulatory framework of the Options Clearing Corporation (OCC), options markets do not permit off-exchange internalization; every single options trade must occur on a registered exchange Wharton PFOF study.

Despite this lit execution mandate, options wholesalers still specialize in capturing retail flow, resulting in a highly concentrated ecosystem where over $80\%$ of retail equity option market volume is routed by just four massive wholesale firms Optiver. Wholesalers navigate the exchange execution mandate by utilizing exchange-provided internalization mechanics, leading to a controversial system that analysts and market participants term “exchange-sponsored PFOF” Optiver.

Marketing Fees and Break-Up Penalties

Because options exchanges recognize that wholesalers control the routing of vast amounts of retail flow, the exchanges compete aggressively to attract these orders. To incentivize routing, exchanges have implemented highly complex fee schedules and rebate programs that heavily favor the wholesalers’ affiliated market makers, specifically through the use of “marketing fees” and “response fees” (often referred to as break-up fees) Optiver.

The mechanics of this system are deeply entrenched. Exchanges collect marketing fees from general, unaffiliated market makers on the exchange Optiver. The exchange then distributes these collected fee pools directly to the specific wholesaler (who is usually affiliated with a market maker on that exchange) that routed the retail order to the venue Optiver. These exchange-sponsored proceeds effectively subsidize the direct PFOF payments that the wholesaler pays out of pocket to the retail brokerages Optiver.

Furthermore, when a wholesaler brings a retail order to an options exchange, they typically enter it into a Price Improvement Mechanism (PIM) auction Review of Financial Studies. The wholesaler proposes a price at which they intend to internalize the trade against their own affiliated market maker Review of Financial Studies. If an unaffiliated market maker participates in the auction and outbids the wholesaler—thereby providing better price improvement to the retail client and “breaking up” the internalized trade—the exchange penalizes the unaffiliated market maker by charging them a steep “response fee” or break-up fee, typically around $\$0.05$ per executed contract Optiver. The exchange then credits this break-up fee directly back to the losing wholesaler Optiver.

This fee structure creates a severe economic headwind for competition. Unaffiliated market makers are forced to pay a massive premium to provide price improvement, while the affiliated wholesalers operate with heavily subsidized economics, insulating them from free-market competition and protecting their ability to internalize flow on the exchange Optiver.

Designated Market Maker (DMM) Priorities

In addition to fee subsidies, options exchanges grant specific queue priority benefits to Designated Market Makers (DMMs) that vastly improve their ability to internalize trades Wharton PFOF study. If a wholesaler acts as a DMM and routes an order to their affiliated exchange while quoting at the NBBO, exchange rules permit the DMM to automatically internalize $100\%$ of the order for up to five contracts, completely bypassing standard price-time queue positioning Wharton PFOF study.

Empirical analysis demonstrates that PFOF-paying DMMs are associated with massive volume advantages. When a DMM pays for PFOF, that specific exchange typically realizes an additional $\$406{,}000$ in monthly trading volume and a $1.1\%$ increase in total market share Review of Financial Studies. Tellingly, these volume increases are concentrated almost entirely in retail-sized trades of fewer than 10 contracts Review of Financial Studies.

The resulting equilibrium in the options market balances price improvement against aggressive institutional defense mechanisms. Larger quoted spreads increase the likelihood that a trade enters a PIM auction, as wholesalers selectively provide price improvement to trades they cannot fully internalize via their automatic DMM allocations Review of Financial Studies. Thus, while the retail options trader still receives nominal price improvement via the auction, the systemic architecture artificially limits true limit-order competition, ensuring that institutional wholesalers maintain unyielding dominance over retail flow execution Wharton PFOF study.

Global Context: Taiwan Stock Exchange (TWSE) Microstructure

To fully appreciate the hyper-specific nuances of the United States RPI and Sub-Penny systems, it is highly instructive to contrast them against international microstructure frameworks, such as the Taiwan Stock Exchange (TWSE) and the Taipei Exchange (TPEx). Unlike the highly fragmented US market—which is characterized by dozens of competing lit exchanges, ATS dark pools, and massive off-exchange wholesaler internalization—the TWSE operates a highly centralized, strictly regulated continuous trading model that addresses retail and institutional segmentation through vastly different mechanisms.

Tiered Tick Size Structures and Price Limits

While the US SEC regulates tick sizes through a binary framework based on TWAQS calculations (either $\$0.01$ or $\$0.005$ for stocks over a dollar), the TWSE employs a complex, multi-tiered tick size system that scales organically with the absolute price of the underlying asset SEC tick-size guide.

The TWSE regular trading system establishes six distinct tick-size brackets for standard equities. If a stock is priced below $\text{NT\$}10$, the minimum tick size is $\text{NT\$}0.01$. For stocks priced between $\text{NT\$}10$ and $\text{NT\$}50$, the tick expands to $\text{NT\$}0.05$ Taipei Exchange trading rules. This scaling continues linearly: $\text{NT\$}0.10$ for stocks up to $\text{NT\$}100$, $\text{NT\$}0.50$ for stocks up to $\text{NT\$}500$, $\text{NT\$}1.00$ for stocks up to $\text{NT\$}1{,}000$, and a massive $\text{NT\$}5.00$ tick size for any security priced at or above $\text{NT\$}1{,}000$ Taipei Exchange trading rules.

This tiered structure ensures that the tick size remains proportionally significant to the overall share price. It inherently prevents the artificial spread widening (tick-constraint) seen in low-priced US equities, while simultaneously preventing negligible queue-jumping in high-priced securities, eliminating the need for complex, retroactive TWAQS evaluations.

For derivative products, the TWSE alters the tick-size constraints to account for leverage and volatility. For Exchange Traded Notes (ETNs), the tick size is fixed at $\text{NT\$}0.01$ when the market price is below $\text{NT\$}50$, and $\text{NT\$}0.05$ when it is $\text{NT\$}50$ or above TWSE ETN rules.

Warrants and Algorithmic Volatility Controls

In contrast to the US model, where Enhanced RPI orders can step-up dynamically to capture flow based on institutional willingness to pay Cboe Enhanced RPI notice, the TWSE relies on strict algorithmic daily price fluctuation limits to balance retail volatility and institutional risk. The limit-up and limit-down prices for call and put warrants are mathematically derived daily, utilizing complex formulas based on the auction reference price and the underlying security’s specific limits TWSE warrant rules.

For example, the limit-up price for a call warrant on the TWSE is calculated as:

\[\text{Limit-up}_{\text{call}} = \text{ARP}_{\text{warrant}} + \left( \text{Limit-up}_{\text{underlying}} - \text{ARP}_{\text{underlying}} \right) \times \text{Exercise Ratio}\]

Here, $\text{ARP}$ denotes the auction reference price at market opening. TWSE warrant rules

During continuous trading sessions (from 9:00 to 13:25), the TWSE matching engine processes orders strictly on price-time priority without any hidden RPI equivalents. Buy orders are prioritized from highest to lowest, and sell orders from lowest to highest. If multiple orders arrive at the exact same price, they are prioritized strictly based upon the chronological sequence of entry, maintaining an absolute adherence to queue position without the complication of hidden, step-up retail liquidity flags Taipei Exchange trading rules.

Block Trading: Institutional Segregation

Instead of relying on off-exchange wholesalers to manage institutional flow away from retail flow, the TWSE utilizes a highly formalized “Block Trading” mechanism to securely segregate large institutional orders from the retail-heavy continuous order book.

To qualify for block trading, a single security order must be for 500 or more trading units, or alternatively, a basket of at least five different stocks with a total dollar amount exceeding $\text{NT\$}15$ million Taipei Exchange trading rules. In the derivatives market, the block trading limits scale by contract type: a minimum of 50 contracts for forex or gold futures, 100 contracts for TAIEX Index Futures, and 200 contracts for TAIEX Options Taiwan derivatives block-trading rules.

Interestingly, while the tick size for regular retail equity trading scales up to $\text{NT\$}5.00$ for high-priced stocks, the tick increment for block-trade price quotes is uniformly fixed at $\text{NT\$}0.01$ Taipei Exchange trading rules. This deliberate architectural choice mirrors the underlying logic of the US RPI system: massive institutional participants trading in bulk require finer pricing granularity to negotiate execution quality without generating outsized slippage. By locking block trades to a one-cent tick regardless of share price, the TWSE provides institutions with the necessary tight pricing increments while safely isolating their massive market impact from the standard retail order book TWSE block-trading rules.

Second and Third-Order Market Implications

The intersection of RPI mechanics, institutional market-making algorithms, and incoming SEC regulatory adjustments generates several profound second and third-order implications for the broader market structure.

The Compression of Wholesaler Profit Margins

The introduction of the $\$0.005$ tick size for tick-constrained NMS stocks represents a direct attack on the profit margins of wholesale internalizers. Historically, a wholesaler could capture a significant portion of a wide $\$0.01$ spread when internalizing an uninformed retail order Davis Polk. By forcibly reducing the minimum spread to a half-penny, the SEC has effectively halved the total available spread capture for over $70\%$ of the market. Furthermore, because PFOF payments sent to brokerages are funded directly by this spread capture, the reduction in minimum pricing increments will inevitably force wholesalers to aggressively renegotiate PFOF rates with retail brokers Wharton PFOF study. If wholesalers can no longer afford to pay high PFOF rates due to compressed margins, the foundational economics of the zero-commission brokerage model may be severely threatened.

The Renaissance of Lit Liquidity and RPI Dominance

As wholesaler margins compress under the weight of the half-penny tick, the economic viability of off-exchange internalization diminishes. Concurrently, the mandated reduction of exchange access fees to a maximum of 10 mils ($\$0.001$) drastically lowers the raw cost of executing a trade on public order books Davis Polk. This dual-pronged paradigm shift is expected to heavily incentivize RMOs to route retail flow back to lit exchanges to interact directly with Enhanced RPI orders. If institutional market makers can no longer profitably pay for order flow off-exchange through bilateral PFOF agreements, they will logically redeploy their quantitative algorithms onto public venues like Cboe EDGX and the NYSE. They will rely on advanced FIX routing, conditional StepUpAmounts (Tag 25025), and Midpoint Peg instructions to aggressively provide price improvement directly on the lit order book, sparking a renaissance of lit liquidity and market transparency Cboe RPI.

The Entrenchment of Options Market Inefficiencies

While the equity market undergoes a massive, pro-competitive tick size compression, the options market structure remains deeply entrenched in anti-competitive exchange-sponsored PFOF and skewed auction mechanics. Because the options market strictly requires on-exchange execution for clearing purposes, the barriers to entry for new, competitive liquidity providers remain exceptionally high due to the systemic burden of break-up fees and DMM priority allocations Wharton PFOF study. Until regulatory bodies address the specific discrepancy of marketing fee allocations and response fee penalties in options markets, the power dynamic will remain heavily skewed toward the four dominant wholesale firms, forcing retail price improvement to occur strictly within the confined, heavily taxed boundaries of PIM auctions rather than through organic, free-market limit order competition Review of Financial Studies.

Conclusion

The price balance between retail investors and institutional market makers is a constantly evolving, highly delicate equilibrium dictated by the intersection of market microstructure, technological routing capability, and aggressive regulatory intervention. Retail Price Improvement orders represent a highly sophisticated engineering solution developed by lit exchanges to reclaim retail flow from off-exchange wholesalers, offering programmatic, sub-penny price improvement to retail takers while simultaneously shielding institutional liquidity providers from the toxic risks of adverse selection.

Through the precise utilization of FIX routing instructions, hidden display flags, pegged price offsets, and advanced conditional step-up algorithms, RPI programs generate tangible economic benefits. They provide critical basis points of price improvement to the retail trader while ensuring institutional execution quality well inside the quoted spread.

However, the architecture of domestic liquidity provision is currently standing on the precipice of a massive structural upheaval. The SEC’s 2024-2026 Regulation NMS amendments—specifically the implementation of the $\$0.005$ minimum pricing increment for tick-constrained stocks and the draconian compression of exchange access fee caps—will fundamentally alter the underlying economics of order routing. By artificially compressing the minimum spread and destroying the traditional maker-taker rebate model, the new regulatory framework directly threatens the profitability of Payment for Order Flow and the dominance of off-exchange internalization. Consequently, the financial ecosystem may witness a historic migration of retail order flow back to public exchanges, where institutions and market makers will be forced to compete openly and aggressively via Enhanced RPI mechanisms, ultimately securing a more transparent, efficient, and balanced pricing environment for the modern retail investor.

References

U.S. Equities: Retail Price Improvement — Cboe Global Markets
BX Retail Price Improvement — Nasdaq Trader
NYSE Retail Liquidity Program rule filing — NYSE
Cboe Launches Retail Price Improvement Program on EDGX — Cboe Global Markets
Cboe Titanium U.S. Equities FIX Specification: RPI Orders — Cboe Global Markets
Cboe BYX Equities Introduces Enhanced RPI Order Type — Cboe Global Markets
Proposed Rule Changes: Cboe BYX Exchange — Regulations.gov
Payment for Order Flow vs. Internalization vs. B-Booking — DayTrading.com
Payment for Order Flow in the United Kingdom — CFA Institute
Payment for Order Flow and the Retail Trading Experience — Wharton Initiative on Financial Policy and Regulation
A Disclosure Gap in the Market for Order Flow — University of Chicago Business Law Review
The SEC Isn’t Mad at PFOF. They’re Just Disappointed — Proof Trading
Institutional Liquidity Demand and the Internalization of Retail Order Flow — University of Warwick
A Deeper Dive into the NYSE Group Retail Liquidity Programs — NYSE
Federal Register Notice, March 24, 2026 — GovInfo
Request for Exemptive Relief
SEC Approves NYSE Retail Liquidity Program — Traders Magazine
Reg NMS Resized: SEC Adjusts Tick Sizes, Lowers Access Fees, and Accelerates Lot Size Amendments — Davis Polk
Order Extending Rule 612(c) Exemptions Until September 30, 2015 — Federal Register
Order Extending NYSE Arca Rule 612(c) Exemption Until June 30, 2018 — Federal Register
Request for Extension of Exemptive Order Under Rule 612(c) — SEC
NYSE Retail Liquidity Program Exemption Extension Letter — NYSE
SEC Adopts Rules on Minimum Pricing Increments, Access Fee Caps, and Better-Priced Order Transparency — SEC
SEC Adopts Rules Modifying Minimum Pricing Increments, Access Fee Caps, and Order Transparency — Sidley Austin
Tick Sizes: A Small Entity Compliance Guide — SEC
SEC Adopts New Regulation NMS Rules on Tick Sizes, Access Fees, and Market Data — WilmerHale
Payment for Order Flow and Option Internalization — Review of Financial Studies
The PFOF You Didn’t Know Existed: Exchange Marketing and Response Fees — Optiver
Trading Mechanism Introduction — Taipei Exchange
ETN Overview and Trading Rules — Taiwan Stock Exchange
Rules Governing Trading of Call and Put Warrants — Taiwan Stock Exchange
2025 Guide to Investing in Taiwan — Taiwan Stock Exchange
Rules Governing Block Trading of Listed Securities — Taiwan Stock Exchange
Taiwan Derivatives Block-Trading Rules — Securities and Futures Law Database

Symbol	Time on Bid Only	Time on Offer Only	Time Both Sides	Total RPI Uptime
SQQQ	\(18.7\%\)	\(16.7\%\)	\(64.3\%\)	\(99.7\%\)
GE	\(19.4\%\)	\(22.4\%\)	\(55.7\%\)	\(97.6\%\)
UVXY	\(30.7\%\)	\(28.2\%\)	\(64.3\%\)	\(30.5\%\) (Non-cumulative)*
NOK	\(36.6\%\)	\(20.7\%\)	\(8.7\%\)	\(66.0\%\)
AAL	\(17.0\%\)	\(29.7\%\)	\(9.3\%\)	\(56.0\%\)
AMC	\(15.7\%\)	\(13.4\%\)	\(8.0\%\)	\(37.0\%\)

Price Per Share	Time-Weighted Avg. Quoted Spread (TWAQS)	Assigned Minimum Tick Size
\(\ge \$1.00\)	\(\text{TWAQS} \le \$0.015\)	\(\$0.005\) (Half-Penny)
\(\ge \$1.00\)	\(\text{TWAQS} > \$0.015\)	\(\$0.01\) (Penny)
\(< \$1.00\)	N/A	\(\$0.0001\)