The subform is just a normal form that has been placed on another (parent) form.

You can use all of the same elements on the "subform" as you do on any other form.

What will be different thought is the view that you choose.  If you choose the "Form View" then you will see all elements as if it was a normal standalone form.  If you choose the "Table View" you will not see unbound elements like labels, buttons, images, etc.  But your combo boxes will be there.

You may want to put some programming on the subform to behave differently if opened as a standalone form.  You want to prevent deletions, new record creations to prevent orphans.  Use @StandAlone command to do that.

Thanks, Bob and Carl for explanations.