In item master, supply source field is mandatory. This same field is also present in site item master. As the supply source for the same item for different sites may be different, therefore it can be defined in site item master.
Its a dropdown field with the following values, Manufactured, Purchased, Stocked, Distributed, Subcontracted.
The stock transfer or distributed from another location in which the product is produced, and the Purchase order to a supplier or to a contract, are an external supplier.
Using a source of supply for in-house production is, the system creates a planned order that is manufactured. Or an item can be stocked by minimum Inventory level.