A card scheme, also called a card network, is the company that runs the rails a card payment travels on: Visa, Mastercard, American Express, Discover, UnionPay and their peers. The scheme connects thousands of issuers and acquirers, routes authorization messages between them, clears and settles the money and writes the rulebook every participant must follow.
Why it matters
Merchants never sign a contract with a scheme, yet scheme rules shape almost everything: interchange categories, chargeback time limits, monitoring programs such as VAMP and the fines that flow from them. When an acquirer tightens terms or drops a merchant, the pressure usually started upstream at the scheme. Knowing which rulebook applies, and reading it before an acquirer quotes it at you, is a real edge in any dispute over fines or thresholds.