Date, Settlement
What it is
MMDD on which the transaction settles between members — set by the network.
DE 15 on the wire
Bitmap position
DE 15 is bit 15 of the primary bitmap: byte 2, mask 0x02. A primary bitmap with only DE 15 present reads 0002000000000000 in hex.
Length
DE 15 is fixed length: always 4 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 4 bytes when the message travels as text (ASCII or EBCDIC), or 2 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.