Merchant Type (MCC)
What it is
ISO 18245 Merchant Category Code of the card acceptor — drives interchange, risk rules, and spend categorisation.
The MCC classifies the merchant's line of business: 5411 grocery, 5812 restaurants, 6011 ATM, 7995 gambling. It feeds interchange qualification, issuer risk and rewards logic, and regulatory blocking (e.g. gambling restrictions by market).
Example values
5411Grocery stores / supermarkets.6011ATM cash disbursement.DE 18 on the wire
Bitmap position
DE 18 is bit 18 of the primary bitmap: byte 3, mask 0x40. A primary bitmap with only DE 18 present reads 0000400000000000 in hex.
Length
DE 18 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.