It's official! Supergirl and The Flash will crossover in the March 28th episode of Supergirl on CBS. Many sources are reporting the news, saying that the CW and CBS universes are being combined, but that's simply not the case. It's obvious to anyone who has actually watched The Flash that CBS's Supergirl universe will be acknowledged as one of the 52 parallel Earth's established in The Flash season 2. Barry will probably make a one-episode visit through a portal, help Supergirl fight a villain, and then return safely to his independent CW universe.
How do we know that this will be the direction the story takes? One word ... Superman.
Superman is a character that exists in Supergirl's world. He's a major influence on the story and that universe, even without being seen. Let's be honest, the existence of Superman would have a drastic impact on any universe, and could certainly not be ignored. That's how we know that Superman does not, or does not yet exist in the CW's shared DC universe, comprised of The Flash, Arrow, and Legends of Tomorrow. If Superman suddenly became common knowledge in the CW DC shows, it would impact the storytelling, and compromise much of their integrity.
Supergirl provides a great example of this. Since her story is so interconnected with Superman's, they had little choice but to acknowledge his existence and incorporate him into the background. That said, they go to great, painful, and laughable lengths to avoid mentioning Superman by name. He's "the big guy", "the guy in blue", "your cousin", or any number of pronouns that are not trademarked. This is because of licensing. If your show mentions "Superman" then your show owes someone a royalty for use of his name. That can get expensive. Showing him would also incur royalties, so we probably won't get a clear, identifiable shot of Supes until well down the road when someone thinks its worth paying for. This has been a hindrance on Supergirl's storytelling, so CW will most certainly not bring this problem home.
Strategically, the crossover is taking place on CBS because they are keen to bring some of CW's passionate, young fans onto their older-skewer network, hoping that they stay around and augment Supergirl's existing audience, but that doesn't mean that a Supergirl appearance on CW is out of the question sometime in the future.
I would point out that both Superman and Batman have been referenced within the CW DC universe already. An early teaser for Legends of Tomorrow featured a voiceover by the show's villain, Vandal Savage, stating "I've seen men of steel die and dark knights fall." That teaser has since been taken offline, but it was the first, obvious reference to both characters from within this universe, albeit from a voice in the future. How far into the future, or if we'll ever experience that future remains to be seen.
In the meantime, Barry Allen has a date with Kara Zor-El on March 28!