Legion theoretically takes place in the world of mutants inhabited by Charles Xavier, Mystique, and even Wolverine. However, the FX series peeks into new corners and expands the already bursting world even further with some original content. For example, is Summerland from Legion in the X-Men comics? The mutant sanctuary is unlike anything that Marvel fans have seen before.
Summerland is like a school or camp for mutants, hidden from the outside world. It is run by Melanie Bird. In a hallucination, Lenny said that Melanie's "secrets have secrets" — was that a sly Avengers reference? Anyway, she pleaded with David not to reveal their location to Division 3, the government agency currently holding Amy Haller hostage, and explained a little about Summerland's origin.
According to Melanie, Summerland used to be a horse ranch. It was built 30 years before the events of Legion by Cary Loudermilk and Melanie's husband Oliver, who currently only exists as the voice on the espresso machine and in the elevator. Melanie wouldn't say whether or not he died. They built it, in her words, "before the Divisions, before anyone knew how fast the world would change." In looking for people to inhabit Summerland, they found Walter — who wanted to hurt people.
Since Melanie launched into this expository story as a response to Syd accusing her of knowing The Eye, I'm guessing that's who Walter is. The "divisions" is a new concept as well — though in X-Men: First Class, the CIA employs Hank McCoy in "Division X."
Summerland definitely seems like a fun alternative to the more traditional education one would get at Xavier's School for Gifted Youngsters. Maybe they're the Ilvermorny to Professor X's Hogwarts. There's more of a West Coast vibe at Summerland for sure.
The fact that the school was built in the 1940s checks out with general history of X-Men. Remember how Magneto is a Holocaust survivor? It'll be interesting to see if Legion connects to the comics in any way. The idea that there are mutant schools everywhere, just doing their own thing, is pretty cool too.