Inventories will just need to reference properties instead of concepts. 

If it involves a single property, then all distinct values from objects of the respective class should be returned. 

If it involves more than one property related to the same class, then all distinct combinations of values from objects of the respective class should be returned.

If it involves more than one property related to more than one class, one or more relationships must specify in the filter how objects from those classes should be joined. Then all distinct combinations of values from joined objects of the respective classes should be returned.