But I would consider these elements as interfaces as wall, even if are used inside of the same assembly.
I mean I would call as interface anything that is directly referred by an assembly constraint.
I do not care if it is in the same document or not. They are still used in the same way.
Perhaps the "interface" is not the best term to describe this particular usage case but I think tat it is better than "element"
which is too generic and can be used for anything.
An other possibility (to avoid the distinction) could be to force users to create a lower level assembly (named part) before creating any geometry.
A new button (new part) should be added for that and only the parts (that are the special assemblies created by new part button) should be allowed to be dragged into a normal assembly.
This is the usual way most commercial CADs (in example Catia) are working.