Merchant category code (MCC)

The codes are standardised in ISO 18245 but maintained in practice by the card networks. 5411 is a grocery store, 5812 a restaurant, 6011 an ATM withdrawal and 7995 gambling and betting. The acquirer assigns the code when the merchant signs up, and it travels with every authorisation from then on.

Four digits decide a surprising amount. Interchange fees vary by category, rewards cards pay bonus points on certain codes, banks block gambling by declining 7995, and a purchase coded as a cash advance can start accruing interest on day one. The classification is also blunt: a superstore selling groceries and televisions gets one code for everything. When ISO approved a dedicated code for firearms retailers in September 2022, a row in a lookup table turned into a political fight.