The Expand on Drill drilling option sets the drilling return set to augment or replace currently displayed dimension members. You can set Expand on Drill for the current document through the data object shortcut menu (Drill , then Drill Options). Expand on Drill can also be set for all subsequently created documents through Drilling preferences.
When Expand on Drill is selected the drilling return set is added to currently displayed dimension members. When Expand on drill is disabled the drilling return set replaces currently displayed dimension members.