MCC blocking is a restriction placed on a payment card so that it cannot be used at certain types of merchant. Every card-accepting business is assigned a merchant category code describing its trade, and the code travels with each request for payment authorization. The issuer or its issuer processor compares the code with a list set for the card and declines the transaction if the category is blocked, or, in the stricter form, if it is not on an approved list.
It is one of the standard spend controls on a commercial card: a fuel card limited to filling stations, or an expense card that refuses gambling and cash withdrawals. Benefit and government cards use it to keep spending to approved goods, and consumer banking apps offer gambling blocks among their card controls. The control is only as accurate as the code. Merchants are sometimes miscoded, and a general retailer's code says nothing about what was bought.