Arboremistus
The Arboremistus is the remix multiverse, the collective set of universes involved in the canon of the remix tree. It was given a name in picotoyunokimyounabouken vol 3.
Real life
Real life is the universe that the Scratch users who made the tree exist in, and the one that Matthew Rockefeller resided in for a period of time. It is a fictionalized version of actual real life, i.e. the universe you, the reader, exist in. In the Arboremistus's real life, the remix world is considered a fictional universe that only exists in media made by remixers.
Remix world
The remix world is the oft-incomprehensible, absurd, and constantly shifting fictional world that most remix media depicts. It and its entire retroactive history was inadvertently created in 2018 by Matthew Rockefeller in their attempt to take revenge on certain OCs.
The remix world's denizens include not just characters who originated there, but OCs as well: characters from other universes who have been brought to the remix world, whether by means of characters in the world creating them, or due to remixers choosing to put them in (though if you think about it, the former is just a way of explaining the latter).
Alternate dimensions
There are alternate dimensions with a similar level of inanity to the remix world, several of which Party Paint Joutaro and Melone traversed.
Pat and Jen's dimension
Joutaro and Melone met Pat and Jen in this dimension, where they discovered that Home Alone 5 was released in 2011, a year earlier than when it was released in the remix world.[1]
Spinning dimension
Joutaro and Melone also encountered a dimension which was identical to Pat and Jen's, except that everything was spinning.[2]
Other fictional worlds
Real life contains plenty of fictional media besides the remix tree, and thus many fictional universes exist which are considered to be part of the Arboremistus, including media from which characters in the remix tree are derived.
Matthew's OC universe
Main universe
Matthew Rockefeller's Warrior Cats OCs exist within a fan universe, although they've also made their way into the remix world. This fan universe is essentially a version of the Warrior Cats setting (four clans of cats existing in a forest with a specific social hierarchy). For a list of characters who exist in this universe, go to Category:Matthew's OCs.
Matthew's OC AU
There is also an AU version of Matthew's fanverse made by someone else in the fandom; one difference is that Poopyface kills Pianolove, who is then revived.
Remix-adjacent worlds
Some Scratch projects and series aren't part of the remix canon, but are still commonly referenced by the remixers, and these are generally considered to have their own universes. The untitled series is a good example, along with the Boyd Simulator series, the Lunchables series, and many of the Pong projects.
Shipping universes
ZiqwerkelSharda and turtlehat paired many characters together in shipping music videos which all exist in their own fanfic universes.