Unique identifier of execution message as assigned by sell-side (broker, exchange, ECN) (will be 0 (zero) for ExecType (150)=I (Order Status)).
Uniqueness must be guaranteed within a single trading day or the life of a multi-day order. Firms which accept multi-day orders should consider embedding a date within the ExecID field to assure uniqueness across days.
(Prior to FIX 4.1 this field was of type int).