Forwarding Institution Identification Code
What it is
Identifies the institution forwarding the message when it is not the acquirer itself (e.g. a processor or gateway).
DE 33 on the wire
Bitmap position
DE 33 is bit 33 of the primary bitmap: byte 5, mask 0x80. A primary bitmap with only DE 33 present reads 0000000080000000 in hex.
Length
DE 33 is variable length (LLVAR): a 2-digit prefix gives the length of the value in digits, from 0 to 11. A 11-digit value is sent as 11 followed by the value. In ASCII the prefix is the bytes 31 31; dialects that pack lengths as BCD send 11 instead.
Size in the message
At most 13 bytes when the message travels as text (ASCII or EBCDIC), or 7 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
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.