Date, Capture
What it is
MMDD on which the transaction data was captured by the acquirer.
DE 17 on the wire
Bitmap position
DE 17 is bit 17 of the primary bitmap: byte 3, mask 0x80. A primary bitmap with only DE 17 present reads 0000800000000000 in hex.
Length
DE 17 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.