Additional Data — Private
What it is
The workhorse private field: networks pack TLV sub-elements here (Mastercard SE/SF data, Visa field 48 usage).
DE 48 is where networks put everything the 1987 base standard never imagined: Mastercard structures it as sub-elements (SE01–SE99, each with sub-fields) covering everything from token requestor IDs to installment data; Visa defines its own usage tables per message type. Never assume DE 48 contents are portable between networks — parse it against the specific network manual.
DE 48 on the wire
Bitmap position
DE 48 is bit 48 of the primary bitmap: byte 6, mask 0x01. A primary bitmap with only DE 48 present reads 0000000000010000 in hex.
Length
DE 48 is variable length (LLLVAR): a 3-digit prefix gives the length of the value in characters, from 0 to 999. 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 1002 bytes when the message travels as text (ASCII or EBCDIC), or 1001 bytes in binary dialects that pack the length prefix as BCD.
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.