Transaction Description
What it is
Free-text description of the transaction, used in payment/transfer messages.
DE 104 on the wire
Bitmap position
DE 104 is bit 40 of the secondary bitmap: byte 5, mask 0x01. A secondary bitmap with only DE 104 present reads 0000000001000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 104 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 104 is variable length (LLLVAR): a 3-digit prefix gives the length of the value in characters, from 0 to 100. A 12-character value is sent as 012 followed by the value. In ASCII the prefix is the bytes 30 31 32; dialects that pack lengths as BCD send 00 12 instead.
Size in the message
At most 103 bytes when the message travels as text (ASCII or EBCDIC), or 102 bytes in binary dialects that pack the length prefix as BCD.
Format ans
Format ans means alphanumeric and special: letters, digits, space and punctuation.
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.