Message Authentication Code (MAC)
What it is
MAC over the message content when a secondary bitmap is present — always the final element.
DE 128 on the wire
Bitmap position
DE 128 is bit 64 of the secondary bitmap: byte 8, mask 0x01. A secondary bitmap with only DE 128 present reads 0000000000000001 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 128 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 128 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.