Settlement Code
What it is
Result of a settlement request (0500/0510 messages).
DE 66 on the wire
Bitmap position
DE 66 is bit 2 of the secondary bitmap: byte 1, mask 0x40. A secondary bitmap with only DE 66 present reads 4000000000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 66 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 66 is fixed length: always 1 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 1 bytes when the message travels as text (ASCII or EBCDIC).
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.