Each document type of Purchase Order is assigned to a Partner Schema, for which the system tries to find all the partner functions assigned to the Vendor Master Record.
Now you can check the Partner Schema assigned under
SPRO --> IMG --> Materials Management --> Purchasing --> Partner Determination --> Partner Settings in Purchasing Documents --> Define Partner Schemas
and assignment of it to the document type in
SPRO --> IMG --> Materials Management --> Purchasing --> Partner Determination --> Partner Settings in Purchasing Documents --> Assign Partner Schemas to Document Types
Here if a certain partner function is marked as mandatory to a schema in Define Partner Schema under Partner Functions in Procedure tab then the system will ask you to maintain it while creating the purchasing document.