Card Acceptor Identification Code
What it is
The merchant identifier (MID) assigned by the acquirer. Appears on statements and in dispute records.
DE 42 on the wire
Bitmap position
DE 42 is bit 42 of the primary bitmap: byte 6, mask 0x40. A primary bitmap with only DE 42 present reads 0000000000400000 in hex.
Length
DE 42 is fixed length: always 15 characters, with no length prefix. A parser reads exactly that many and moves on, so shorter values are left-aligned and padded with trailing spaces.
Size in the message
At most 15 bytes when the message travels as text (ASCII or EBCDIC).
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.