Amount, Settlement Processing Fee
What it is
DE 30 fee converted to the settlement currency (C/D sign plus 8 digits).
DE 31 on the wire
Bitmap position
DE 31 is bit 31 of the primary bitmap: byte 4, mask 0x02. A primary bitmap with only DE 31 present reads 0000000200000000 in hex.
Length
DE 31 is fixed length with no length prefix: always the C or D sign plus 8 digits, 9 characters in all. Amounts are right-aligned and padded with leading zeros.
Size in the message
At most 9 bytes when the message travels as text (ASCII or EBCDIC), or 5 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.