Reserved — ISO
What it is
Reserved for ISO use.
DE 105 on the wire
Bitmap position
DE 105 is bit 41 of the secondary bitmap: byte 6, mask 0x80. A secondary bitmap with only DE 105 present reads 0000000000800000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 105 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 105 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 ISO” means for your parser
ISO 8583:1987 keeps DE 105 for later versions of the standard, so a 1987 message should not carry it. Your parser still needs its LLLVAR ans definition to step over the field if it appears. A counterparty that sends it is using a later version or a private extension, and their interface specification defines the content.
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.