An authorization request reaches the issuer through the card network carrying the amount, the merchant category, the country and whatever authentication result came with it. The issuer checks account status, available balance or credit line, and its own fraud model, then answers.

For the merchant, that answer is the end of the matter. Support teams asking what is issuer decline code 51 are asking about an empty account rather than about the bank itself. Resending an identical request does not change an issuer's view; only a reason code signaling a temporary condition makes another attempt worth making, which is why retry logic depends on reading the code rather than trying again blindly.

The issuer also funds the transaction — advancing the money on a credit card and billing later, or debiting a balance it already holds on a debit card.

Interchange is the core of it: a fee flowing from the acquiring side to the issuing side on every card sale, set by the schemes and capped by regulation in several markets. Cards with richer rewards carry higher interchange, which is why a single merchant pays different rates for two cards that look identical at the counter.

Around interchange sit annual and monthly card fees, interest on revolving balances, FX markups on foreign spending and cash-withdrawal charges. The mix explains the product: a card built to earn interest is designed differently from one built to earn interchange.

Most consumer fintechs handing out cards are not issuers in the legal sense. A BIN sponsor holds the scheme licence and the issuer status, an issuer processor runs the authorization logic and cardholder ledgers, and the program manager owns the app and the customer.

Where the balance behind the card is stored value rather than a bank deposit, the regulated product is e-money and the card behaves as a prepaid card whatever the branding says. That distinction decides which licence the program needs and who answers to the regulator when something breaks.

Issuer systems fail. The schemes cover the gap with stand-in processing, where the network approves low-value transactions against limits the issuer agreed in advance. The merchant sees an ordinary approval, and the issuer absorbs whatever came through once its systems return.