Time in Force

TIF / Order Type	FIX encoding	Behavior
Day / Good Till Cancel (GTC)	59=0	Default for Limit Orders and Quotes. 59=0 (Day) behaves as good-till-cancel for the trading day — do not send 59=1. Rests on the book until filled or canceled; expires at end of day.
Immediate or Cancel (IoC)	59=3	Fills as much as possible on arrival; any remainder is canceled immediately. Never rests on the book. Always applied to Market Orders; optional for Limit Orders.
Fill or Kill (FoK)	59=4	Must fill completely on arrival (in one or more executions); if it cannot, the entire order is canceled. Never rests on the book.
All or None (AoN)	ExecInst=G (18)	Like FoK, but if it cannot fill in full on arrival it rests on the book until it can be filled in full.
Single Fill or Cancel (SFC)	ExecInst=Z (18)	Like IoC, but must fill the entire quantity in a single execution off the top of book — no partial fills, no sweeps. If the order cannot be fully filled in one fill on arrival, it is canceled immediately.

AoN and SFC are order types encoded via ExecInst (18); when ExecInst is set, TimeInForce (59) is ignored.