What is the difference between a PCF and an SCF?

The method is the same and the standards are the same. The difference is the unit. A PCF is per physical unit of a product. An SCF is per unit of a service, which has no physical output, so the footprint is defined by a functional unit the supplier chooses, for example per user per year or per tonne-kilometre. Comparing two SCFs is only valid when the functional unit matches.