You create a purchase order (PO) manually and you expect the Purchasing Unit to default automatically according to the Product Category as per the work Distribution rules defined for the work category Operational Purchasing (work center Organizational Management). However the defaulting does not occur according to the Product Category.
The Product Category setting in work category Operational Purchasing is not used for manual POs.
So it will have effect only for the automatic creation of Purchase Requests (and follow-up POs if it is configured) via Planning, Shopping Carts, etc... So the Purchase Requests will contain the right Purchasing Unit.
The setting is only to distribute the workload among the purchasing units (purchasers), not to automate the filling of POs.
KBA , SRD-MD-OM , Organizational Management , How To