Exactly what it sounds like. Anyone with a differing design... except for those who have an Earth-origin, but even those might not be originating from the Earth you know, isn't from the Cybertron... the reality you call home.

[Jetfire had finally gone to look away from considering the corridors to focus on Breakdown, shrugging slightly.]

That, truly, is the largest issue with anyone of us going home, rather than the lack of a method currently.