Source of supply is a exactly what it says...it will be the supply source for fulfilling the needs of a receipt element. In case of in-house production it can be either PPM or PDS. In case of external procurement it can be a transportation lane for the vendor.
Planning versions give you test-beds to simulate various planning scenarios. So each planning version can store different transactional data sets. You can then choose the best simulation and merge it to the active planning version 000.
