Controlling stock purchases

Purchasing would like to control the other departments from entering purchase requisition using specific item numbers. For example, if item A then use order type A. If item B, they try to enter then system should prompt an error. We are testing using GL Class Code to group the items and setup DMAAI by order type and the GL Class Code to prevent the users from saving the wrong combination of purchase requisition. But we encountered error when testing stock purchase requisition whereby even though we did not set the GL Class Code into DMAAI for the stock purchase order