Ather,

 

It is not possible for you to require a field to be populated by a user.   You are however able to monitor fields so that if they are not populated, the appropriate update can be applied.

 

For the category, sub-category, manufacturer, and locations fields, you can use the associated maintenance function where the "Show Unassigned" check box shows you those stock codes that have no value assigned and allows you to update that value.  Alternately, you can create a user defined report and specify any field that you wish to monitor such that the report lists those stock codes.