Reserved — National
What it is
Reserved for national use. ISO 8583:1993 assigns it to the authorization life cycle code (n 3).
DE 57 on the wire
Bitmap position
DE 57 is bit 57 of the primary bitmap: byte 8, mask 0x80. A primary bitmap with only DE 57 present reads 0000000000000080 in hex.
Length
DE 57 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.
What “reserved for national use” means for your parser
DE 57 is left to national bodies: a domestic scheme or network may define it for traffic within one country. ISO fixes only the envelope (LLLVAR, up to 999 characters), so the layout comes from that network's specification. A parser that does not know the national layout should keep the value as an opaque string and pass it on unchanged.
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.