What is the difference between an ERP and SCM software?

ERP systems hold the master data and the transactional record: orders, invoices, stock movements, supplier contracts. SCM software sits above the ERP and produces the plans that drive operational decisions: how much to forecast, how much to buy, when to replenish, how to allocate stock across the network. The ERP is a system of record; SCM software is a system of decision.