Response Code
What it is
DE 39 is the single most-read field in a response. 00 (and 10/11 for partial/VIP approvals) mean approved; everything else is a decline or error class. Codes divide into retryable infrastructure failures (91 issuer inoperative, 96 system malfunction), hard declines (04 pick up, 05 do not honor, 14 invalid card, 54 expired), and soft declines that may succeed on retry or with SCA (51 insufficient funds, 65/1A soft decline requiring authentication).
Networks extend the base set: Visa and Mastercard each publish their own response-code tables which map onto (and beyond) the ISO values. The full searchable table lives in the Response Code Reference tool.
Example values
00Approved.05Do not honor — generic hard decline.51Insufficient funds — soft decline.91Issuer or switch inoperative — retryable.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.