You need to pay attention when will create a Purchase Order for different Product Categories.
Is not possible to create one purchase order with items that have different logical systems, and when you select the product category, the source system is taking into account.
If the product categories involved in the PO items have different source systems assgined, an error will be issued:
BBP_PD 176 "Purchase order contains items with different logical systems"
To check the logical system assigned for product Category:
-> Supplier Relationship Management
-> SRM Server
-> Technical Basic Settings
-> Define Backend System for Product Category