Reserved — Private (Network Data)
What it is
Private use. Mastercard: network data (financial network code, Banknet reference). Visa: V.I.P. private-use field (network ID and message reason data).
DE 63 on the wire
Bitmap position
DE 63 is bit 63 of the primary bitmap: byte 8, mask 0x02. A primary bitmap with only DE 63 present reads 0000000000000002 in hex.
Length
DE 63 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 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.