TAS lets funds that are benchmarked to settlement guarantee they will match it. You trade a differential — zero, +1 tick, -2 ticks — and the actual price is stamped in after daily-settlement is computed.
It is heavily used by index rollers and commodity funds, and TAS volume in the days around a roll is a decent tell for where passive money is moving.
Example: buying 50 CL at "TAS -1" fills you one tick below whatever crude settles at. If settlement prints $79.42, your fill is $79.41 on all 50.
Related: settlement-price, daily-settlement, roll-date, block-trade, banging-the-close