>In movies, they call it "cast in order of appearance". We could derive some better term using that as a guideline. How about older/younger objects, regarding their order of birth?

Yes...I'm proposing a new idea here, there will be an initial ordering of same-level objects called TOrder, based on temporal (like birth) order. Unless overridden by ZOrder (which really should be strictly limited to stacking in the Z-plane, the TOrder will be the primary instantiation order....