For a particular Job mark the Operation as Subcontract and in subcontract tab select the material and quantity
Po creation is usually for vendor for costing purpose. eg you are sending 100 Kg material for cutting then mention in PO the unit cost of cutting agreed with vendor.
Perform Subcontract Shipment Entry to notify system that raw material has gone outside and Reciept entry against the PO to mark inwards movement of material.
Job absorbs the Subcontract cost and Finance can then AP Invoice for vendor(sub contract) and AR invoice for customer can be created in system
Hope this clarifies issue