Receiving Institution Identification Code
What it is
Identifies the institution the message should be routed to when it is not implied by the PAN — e.g. in transfers.
DE 100 on the wire
Bitmap position
DE 100 is bit 36 of the secondary bitmap: byte 5, mask 0x10. A secondary bitmap with only DE 100 present reads 0000000010000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 100 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 100 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.