ExecID <17> field – FIX 5.0 SP2 – FIX Dictionary
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 <17> field to assure uniqueness across days.
(Prior to FIX 4.1 this field was of type int)