Bit Map, Extended
What it is
ISO 8583:1987: "Bit map, extended", b 1 — a single bit reserved to signal a further bitmap; 1987 defines no data elements past 128. Some implementations treat a set bit 65 as the flag for a third (tertiary) 64-bit bitmap covering DE 129–192, and the ISO 8583 parser and bitmap tools here read it that way. Essentially unused in production networks.
DE 65 on the wire
Bitmap position
DE 65 is bit 1 of the secondary bitmap: byte 1, mask 0x80. A secondary bitmap with only DE 65 present reads 8000000000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 65 has a primary bitmap whose first hex digit is 8 or higher.
Format b
Format b means binary: raw bytes, often shown or sent as hexadecimal text.
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.