Purchase order vs requisition

A purchase requisition is an internal request to buy something. It is issued by the requesting department and approved by procurement. A purchase order is the external document sent to the vendor; it is a binding commitment to buy at the agreed terms. The requisition is internal; the PO is external.

From requisition to PO

Once approved, a requisition can be converted into a purchase order. The conversion copies the line items and the parties; the requisition's requester and department fields are dropped (they were internal and are not part of the external commitment). The conversion is a true_conversion in the conversion graph.

From requisition to RFQ

For larger purchases, a requisition can be converted to an RFQ first, then to a vendor comparison, then to a PO. The requisition → RFQ step is also a true_conversion; the RFQ → comparison step is an aggregation.