Scheme rules is the general term for the rulebook of a payment scheme, and in the card business it means the same thing as card network rules: the requirements that Visa, Mastercard and other networks impose on the issuers and acquirers that participate. The main entry describes what those rules cover, from acceptance and authorization to disputes, fees and penalties.
The phrase is common in Europe and other regions where card networks are called card schemes. Merchants are not usually members of the scheme, but they are bound to the rules indirectly through their acquiring agreement, and a breach can lead to fines passed down by the acquirer or to the loss of card acceptance.
The term also applies beyond cards. Any payment scheme has rules that define message formats, timelines, liability and how disputes between members are settled. Examples are the SEPA rulebooks published by the European Payments Council, the Nacha Rules for the US ACH network and the rules of national instant payment schemes. Card networks often call their own documents operating regulations. Scheme rules are private contracts, not law, and they sit alongside the regulations that apply to payment providers in each country.