A protocol is the versioned, schema-validated API contract – the actions callers invoke (assess, authorize, capture, refund) and the shape of each request and response. A backend is the binding from that protocol to a real provider: credentials, an authentication strategy, and the declarative mapping rules that translate the contract into the provider's wire format.

Because the two are separate, you can point one protocol at a different backend – or run several in parallel – without the caller ever knowing.

→ Protocols vs. backends