Supplier performance management (SPM) is the ongoing process of measuring, evaluating and improving how suppliers and service providers perform after award.
The cycle. Set expectations (metrics, targets, owners), measure performance, review results on a regular cadence, agree actions, follow up on whether they worked. Stopping at measurement is reporting, not management.
Where it sits. Evaluation before award is supplier evaluation, SPM starts with the contract. Performance measurement produces the numbers, SPM acts on them. Supplier relationship management (SRM) is broader and focuses on developing strategically important suppliers. SPM supplies the data it relies on.
For services. There are no unit counts or delivery dates to track. Instead you measure acceptance of deliverables, milestone adherence, team continuity, responsiveness and stakeholder assessment. Part of the evaluation is therefore a human judgement and not a system value.
Terminology. "Vendor performance management" is a synonym. "Vendor" is used more often for service providers and external workforce, "supplier" for procurement in general.