Amount, Net Settlement
What it is
Net settlement position, C/D sign plus 16 digits (17 characters on the wire) — used in reconciliation (0500) messages.
DE 97 on the wire
Bitmap position
DE 97 is bit 33 of the secondary bitmap: byte 5, mask 0x80. A secondary bitmap with only DE 97 present reads 0000000080000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 97 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 97 is fixed length with no length prefix: always the C or D sign plus 16 digits, 17 characters in all. Amounts are right-aligned and padded with leading zeros.
Size in the message
At most 17 bytes when the message travels as text (ASCII or EBCDIC), or 9 bytes in binary dialects that pack digits as BCD.
Format x+n
Format x+n means a sign character, C for credit or D for debit, followed by numeric digits.
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.