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).
DE 11 on the wire
Bitmap position
DE 11 is bit 11 of the primary bitmap: byte 2, mask 0x20. A primary bitmap with only DE 11 present reads 0020000000000000 in hex.
Length
DE 11 is fixed length: always 6 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 6 bytes when the message travels as text (ASCII or EBCDIC), or 3 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
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.