Just check out the summary and bios pages!
This AU is pretty much an open sandbox with a set of guidelines and major plot points, much like tabletop RPGs. There are constants and important info in the summary, and character relevant info of the most common characters in the bios. The core of the story revolves around the canon characters, but a lot of people have made branching side-stories involving OCs!
You don’t have to know anything besides the core information in the summary and bios pages to get the gist of most fics out there.
A lot of them are tiny, self-contained arcs that can be very ambiguous when it happens in the timeline. Which is very easy to follow, since the timeline itself can be divided into two major sections: during Mabel’s lifetime and Dipper is a young fledgling demon with familiar characters and a modern setting–and after Mabel’s lifetime, which each story creates its own setting as to how far in the future it is (how technology has changed, how stable Dipper is at the time, etc).
Hope this helps, and welcome to TAU!

