Reserved — Private
What it is
Private use. Mastercard: record data for file updates. Various switches: terminal address/extended data.
DE 120 on the wire
Bitmap position
DE 120 is bit 56 of the secondary bitmap: byte 7, mask 0x01. A secondary bitmap with only DE 120 present reads 0000000000000100 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 120 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 120 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 private use” means for your parser
DE 120 is for private use: the network, or the two parties exchanging messages, define what goes inside, often as sub-fields or tag-length-value data. ISO fixes only the envelope (LLLVAR, up to 999 characters). Treat the value as opaque unless you implement that network's specification.
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.