It stands for mid-point liquidity order; a limit order to buy (sell) a security (or a tradable asset) that is not displayed and automatically executes at the mid-point of the protected best bid or offer (PBBO)- a working price is filled at the lower (higher) of the midpoint of the protected best bid and offer (PBBO) or its limit price.

This type of order interacts with incoming orders, including other same-type orders, but not an incoming add-liquidity only (ALO) limit order. An MPL order will not execute under specific market conditions- such as a locked or crossed market. Otherwise, all eligible MPL orders execute, excluding MPL-ALO orders and other invisible/ hidden interest eligible to execute at the midpoint of the PBBO. Furthermore, an MPL order is not eligible for manual executions, including openings, re-openings, or closing trades.

An MPL order to buy (sell) must be designated with a limit price in the minimum price variation (MPV) for the security and is eligible to trade, only if, at the midpoint of the PBBO, it is at or below (above) the limit working price of the order. In case the protected best bid (PBB) or the protected best offer (PBO) are not available, or the PBBO is locked or crossed, all MPL orders, incoming and resting, will be queued for eligibility- until the time a PBBO that is not locked or crossed.

This order is also known as a mid-point passive liquidity order.