Response Code
What it is
The action/result code of the request: 00 approved, 05 do not honor, 51 insufficient funds, 91 issuer unavailable…
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. Visa groups declines by whether a reattempt may succeed: category 1 never (04 pick up, 14 invalid card, 41/43 lost/stolen), category 2 later (51 insufficient funds, 65, 91 issuer inoperative, 96 system malfunction), category 3 only with corrected data (54 expired card), and category 4 generic codes such as 05 do not honor, which may be reattempted within Visa’s reattempt limit. Mastercard 65 and Visa 1A are soft declines that ask for strong customer 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 decline (Visa category 4: reattempts allowed within the limit).51Insufficient funds — Visa category 2, may succeed later.91Issuer or switch inoperative — retryable.DE 39 on the wire
Bitmap position
DE 39 is bit 39 of the primary bitmap: byte 5, mask 0x02. A primary bitmap with only DE 39 present reads 0000000002000000 in hex.
Length
DE 39 is fixed length: always 2 characters, with no length prefix. A parser reads exactly that many and moves on, so shorter values are left-aligned and padded with trailing spaces.
Size in the message
At most 2 bytes when the message travels as text (ASCII or EBCDIC).
Format an
Format an means alphanumeric: letters and digits.
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.