Format: nLength: 6Fixed length
DE 11
System Trace Audit Number (STAN)
What it is
The STAN is assigned by whoever initiates the message and stays constant through the conversation: the 0110 response, and any 0400 reversal of that transaction, carry the same STAN. Hosts key their timeout/reversal logic on STAN + DE 7 + terminal identifiers.
It rolls over at 999999, so it is only unique within a window — which is why matching always combines it with the transmission timestamp and, where present, DE 37 (RRN).
Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.