I remember Comyet once compared multiverse with HTML and CSS. I actually like this comparison cause it fits pretty well but one thing just came in my mind.
Undertale was made with GML and this language uses OOP. I don't know anything about this language and game programming in general but I think that they probably use classes for some stuff. And classes might have hierarchy.
For example multiverse might be base class and undertale might be child class that has 2 child classes like underfell and underswap (for example) and they also might have 2 child classes like swapfell and fellswap (hope that you got the point) and almost all aus might have some kind of connections like this.













