This field must contain a value according to the selected replenishment system before the system can create a supply order for the line.
Note: This is not the case if the replenishment system is Prod. Order.
You can use the AssistButton in the field to look up in the relevant list and select where to supply from:
If replenishment system is Purchase, the field looks up in the Item Vendor Catalog window.
If replenishment system is Transfer, the field looks up in the Location List window.