What are the "specs" of the afterlife world? Regarding the afterlife world we see in the Angel Beats! anime, I have many questions, but I think we can pack all of them as "what are the specs of the afterlife world?":


Are habitants limited to the school grounds? Can they, say, go somewhere really far away?
Is this world based on an actual place in the living world? For instance, does this school actually exist somewhere?
Do all individuals that die with regrets end up here? Surely millions of people die this way, and I'm pretty sure that we never see more than a few thousands.
Are there other afterlife worlds? Or everyone ends up in the same place?
So for this bunch of questions I am going to use spoiler blocks as they are explained in the anime.

Limitations to school grounds:


   If I recall correctly, Yuri mentioned something like "behind the school boundaries there is a large and wide area of nothing but grass" and that she tried to escape, but was not able to.


Based on any actual place:


   In general, buildings are partly based on real buildings or generic. I would guess this school is a rag rug of multiple buildings mixed, excluding the underground facility of course ;)


Why not everyone / other afterlife worlds:


   Well, everyone who dies with regrets seems to go to a certain place, where they can get over these regrets and live a new, untainted life. Also, most of the students are mere "filler material" / "background noise" to create a perfect environment. They were created by the "Programmer". This person wanted to create a perfect world for students to clear their regrets. Thus the school is only for students. This means a part of Japanese highschool students get to this special place created for them. For other people dying with regrets, there "must" be a different place to go.