The bill of manufacturing requires specific part numbers to insure the engineering integrity of each parent item.  If a particular component can be purchased by revision and there is a qualitative difference among revisions, each revision must be represented by a unique item ID so that the correct revision can be assigned to the BOMs where it is used.  If a component’s revisions are interchangeable, meaning there is no qualitative difference from on revision to another, a single item ID can be used.