User blog comment:CITRONtanker/Dimensional Doomsday. A Season 3 episode idea/@comment-27873981-20171001153901/@comment-27480112-20171001224508

Those are different dimensions we're talking about. It'd be best to describe those as "universes".

And besides, these dimensions were actually once part of the IaLR Earth only to get split into their own pocket world!