File Update Code
What it is
Action for file-update messages (0300): add, change, delete, inquiry.
DE 91 on the wire
Bitmap position
DE 91 is bit 27 of the secondary bitmap: byte 4, mask 0x20. A secondary bitmap with only DE 91 present reads 0000002000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 91 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 91 is fixed length: always 1 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 1 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.