There is documentation describing the pro's and cons for both options:

https://docs.mendix.com/refguide/generalization-and-association/

You could have a generic PurchaseOrder storing the information of an order, but depending on the department there are specific details that vary. You could create a specialisation of PurchaseOrder to capture the different variations or create an associated entity PurchaseOrderDetails and associate that with one-to-one association.