Supplemental Liquidity Provider
A Supplemental Liquidity Provider (SLP) is a registered market participant on the NYSE that commits to electronically posting displayed limit orders in assigned securities for a specified minimum percentage of the trading day in exchange for enhanced rebates on liquidity-adding trades, serving as an electronic complement to the Designated Market Maker system.
The NYSE introduced the Supplemental Liquidity Provider program in 2008 as part of its hybrid market structure redesign following the Archipelago merger. The program was designed to attract high-frequency and electronic market-making firms that could inject continuous electronic liquidity into NYSE-listed securities — specifically to improve quote depth and reduce spreads in securities where the DMM alone might not maintain sufficiently competitive two-sided markets.
SLPs are assigned to specific NYSE-listed securities, similar to DMMs, but their obligations are less onerous. An SLP must post a displayed limit order on one side of the inside market — either the bid or the offer — for at least 10 percent of the trading day in each assigned security. This minimum quoting obligation is the primary regulatory commitment. Unlike DMMs, SLPs do not have obligations related to auction facilitation, manual intervention, or maintaining a specific maximum bid-ask spread.
In exchange for meeting the quoting obligation, SLPs receive an enhanced maker rebate from the NYSE for trades in which they provide liquidity — typically a higher rebate per share than is available to non-SLP participants. In the NYSE fee structure, exchanges charge 'takers' (those who remove liquidity by hitting posted quotes) and pay 'makers' (those who post displayed limit orders that get hit). The SLP rebate enhancement is the economic incentive that makes the obligation commercially attractive to high-frequency firms.
The economic logic of the SLP program is that the rebate enhancement pays for the commitment to be consistently present in the market, even during volatile periods when market-making is most risky. Without this commitment mechanism, electronic market makers would withdraw their quotes during volatile conditions precisely when their liquidity is most needed — a dynamic sometimes called 'ghost liquidity,' where quoted depth disappears under stress.
For investors, the SLP program contributes to tighter spreads and deeper quoted markets in NYSE securities. The competitive interaction between multiple SLPs and the DMM in each security generates a more competitive quoting environment than a single liquidity provider could maintain. Critics of maker-taker fee models, of which the SLP rebate is a part, argue that these fee structures create conflicts of interest for broker-dealers who route orders to maximize rebates rather than to achieve best execution for clients.