Board Thread:General Discussion/@comment-16581186-20161115131009/@comment-3423279-20161115203848

Also, you should look up the Vixen series, Mari McCabe appears in that and it is confirmed to appear in the same setting as Arrow and Flash as evident by their appearances in it and later on Oliver referencing it, that's the reason it was included.

Plus, it was always confirmed to take place in the same world as the other two shows.