Hi,
Why user will do mistake - you should design your Purchase Order Document Types and restrict "Allowed Item Category" for each Purchase Order Document Type.
You can define PO Document Type in following path:
SPRO -> Materials Management -> Purchasing -> Purchase Order -> Define Document Types
&
Restrict each Purchase Order Document Type by allowing item category in each process in "Allowed Item Category" segment
For example:
PO Document Type------------"Allowed Item Catagory"-----------------Result
NB----------------------------------------BLANK-------------------------------------Only use for material procurement
ZKNB------------------------------------ ONLY allow K---------------------------Only use fro Consignment
ZSTO-------------------------------------ONLY allow U---------------------------Only used for Stock Tran.Order
ZZUB-------------------------------------ONLY allow L----------------------------Only use for Subcontracting
Also,talk to Basis Consultant and with help of authorization,you can control Purchase Order creation based on Purchase Order Document Type.
Regards,
Biju K