Trigger price

The condition attached to a pending instruction. Until the market reaches the level the order rests and does nothing; on reaching it the order is released into the market as an instruction of its own. A stop order releases a market order, so it names no price for the eventual fill at all, and the level that activates it is the only price it specifies.

Two events therefore have to be held apart, because platforms present them as one. The trigger is a condition being met. The fill is a transaction, and it happens afterwards, at whatever the market offers at that moment. In an orderly market the two are close enough to look simultaneous. In a gapping or fast moving market they are not, and the difference between them is slippage rather than a failure of the order.

A stop limit order carries two prices instead of one, a trigger and a limit, and the distance between them is the whole design of the order: it converts an uncertain fill price into an uncertain fill, since a limit the market has already passed leaves the instruction unexecuted. Neither certainty is available at the same time as the other, on any venue and in any market, because the two are the same fact stated from opposite ends.