Definitions
While not too important, there is a distinct difference between a Branched Universe and a Parallel Universe to be aware of as well as other terms.
Space
Space is what is inside of a Universe filled with Galaxies, Solar Systems, Planets, Moons and more. Planets are large bodies that usually orbit a star within a Solar System while Moons orbit them, each character has a home planet, some with access to other planets and more.
Universes and Branches
A Universe is a single reality with a single timeline, refered to as spacetime. Time cannot be altered as everything that did happen always did, however new braches of alternative realities known as Branched Universes can occur in theory if one was to go back in time and create things they would instead just create a new number of branches. Spacetime within a Universe is endless, though there is an origin. A lot of characters are from alternate branched Universes to each other where at some point in the unknown past, their timelines branched apart from each other.
Parallel Universes
A set of branched Universes can be considered part of a Universe Tree sharing the same root, however Parallel Universes are completely different being from a whole new tree or something else entirely. Parallel Universes can work entirely different from each other, in Hell for example, time itself works in a different way and the Branched Universes of Hell constantly mix in and out of eachother creating a chaotic world. There are an infinite amount of Parallel Universes that exist within the Multiverse.
Pocket Universes
Pocket Universes are small limited Universes that are usually bound within a parent Universe, the way they work is usually the same as the Parent Universe but they can vary a bit. Pocket Universes can also be moved to other Universes or cast out into the Void. Over time a Pocket Universe is capable of developing into a fully fledged Parallel Universe and is a possible origin for the many Parallel Universes that exist.
Multiverse
The Multiverse is a term that means, absolutely everything. There is only one Multiverse as it is everything, another Multiverse would mean another everything which makes no sense and would just be a bigger Multiverse.
The Void
The Void is nothingness or empty space. The Void isn't a place that can be visited as it isn't really a thing, this is just what nothingness is called, to be lost in the void would likely mean being in a small bubble of reality (aka a Pocket Universe) drifting through nothingness. The Void Element of Magic isn't nothingness itself, but more the pull of nothingness, powerful Void magic is able to consume reality to a point of no return.