Message Authentication Code (MAC)
What it is
MAC over the message content when only the primary bitmap is present — integrity protection between nodes.
The MAC proves the message was not altered in transit: an ISO 9797 (X9.19 retail) MAC computed over the message under a MAC key shared between the two nodes. When a secondary bitmap is present the MAC moves to DE 128 so it stays the last element.
DE 64 on the wire
Bitmap position
DE 64 is bit 64 of the primary bitmap: byte 8, mask 0x01. A primary bitmap with only DE 64 present reads 0000000000000001 in hex.
Length
DE 64 is fixed length: always 8 bytes, with no length prefix. A parser reads exactly that many and moves on.
Size in the message
Exactly 8 bytes of binary data. Interfaces that carry binary data as text send each byte as two hex characters, so the value takes 16 characters there.
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.