Procure-to-pay is the operational side after award: purchase order, receipt or service confirmation, invoice matching, payment.
Flow. Requisition → order → receipt/confirmation → invoice matching → approval → payment.
Distinction from S2C. Assumes a contract already exists.
What's different for services. A service confirmation replaces goods receipt, matched against a SoW milestone.
Metrics. Cycle time, e-invoice share, touchless processing rate, payment delay.