It stands for midpoint liquidity (MPL) adding liquidity only order (ALO); an MPL order that is designated with an ALO modifier. It is a non-displayed midpoint pegged limit order that is executable only at the midpoint of the national best bid and offer (NBBO), and down to 3 decimal places.

MPL ALO orders rest and remain valid for all sessions but do not take part in any auctions. On arrival, an MPL-ALO order to buy (sell) will trade against (be matched with) resting orders to sell (buy) with a working price below (above) the midpoint of the NBBO (the protected best bid and best offer (PBBO)), but not those priced at the midpoint of the PBBO.

Marketable MPL ALO orders rest in a venue (and do not cancel back), waiting for execution only if the order is able to add liquidity (ALO).