Board Thread:The Flash discussions/@comment-9346948-20160210010105/@comment-4324417-20160210020828

No, it's not the "Arrowverse Multiverse", it's the multiverse. The Arrowverse doesn't belong to one universe, and two, Earth freaking 2 is an integral part of The Flash. That particular Earth is a big part of The Flash, Supergirl isn't.

Not to mention, John Wesley Shipp's Flash earth is also apart of the same multiverse.

http://comicbook.com/2016/02/10/supergirl-cameos-in-the-flash-tonight-and-what-that-means-for-th/