System Trace Audit Number (STAN)
What it is
Rolling per-message number assigned by the initiator, echoed in the response — the primary request/response matching key.
The STAN is assigned by whoever initiates the message and stays the same within one exchange: the 0110 response and any repeat (0101) carry the request’s STAN. A 0400 reversal is a new exchange — the original STAN travels in DE 90, although some hosts reuse it in DE 11 too. 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.