A must be filled (MBF) order is a trade that must be executed due to expiring options or futures contracts on those exchanges. Many MBF orders are filled on the third Friday of each month because index options often expire on those days each month. MBF orders have the advantage of being exempt from short sale rules.
MBF orders must be put into the system by 5:30pm on the day before the expiration date. These orders then must be filled immediately at the opening of options or futures exchange on the following day, which is the expiration day. These orders must be filled at the opening price of the options or futures exchange.