Account Identification 1
What it is
The "from" account in transfers and inquiries — account number or IBAN, per network convention.
DE 102 on the wire
Bitmap position
DE 102 is bit 38 of the secondary bitmap: byte 5, mask 0x04. A secondary bitmap with only DE 102 present reads 0000000004000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 102 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 102 is variable length (LLVAR): a 2-digit prefix gives the length of the value in characters, from 0 to 28. A 12-character value is sent as 12 followed by the value. In ASCII the prefix is the bytes 31 32; dialects that pack lengths as BCD send 12 instead.
Size in the message
At most 30 bytes when the message travels as text (ASCII or EBCDIC), or 29 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.