A Purchase Order can be created with the reference to the following:-
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Source List
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Material Master
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Contract
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Goods Receipt
In SAP MM, a Purchase Order can be created with reference to several documents, including Contracts, Purchase Requisitions, and Requests for Quotation. A Contract is a valid reference document that outlines long-term purchasing agreements. Source List and Material Master are master data, not transaction documents that can serve as references. Goods Receipt happens after PO creation.
A Purchase Order in SAP MM can be created with reference to an existing Contract (an outline agreement), pulling in the agreed price and terms automatically. Material Master and Source List are master data/reference information used to help create a PO but aren't themselves the 'reference document' a PO copies from, and a Goods Receipt happens after a PO exists, so it can't be a source reference for creating one.