The Omniverse, within the context of Infinitypedia, is defined as the set of all universes conceivable. As a result, you can fit any sort of universe you can think of in the Omniverse, and it would essentially be canon to the Omniverse.
A multiverse is a group of universes and/or multiverses, either abstractly imagined or existing as a physical structure in the Omniverse. The concept of multiverses can be divided into two major subclasses: logical multiverses and physical multiverses.
A logical multiverse is a group of universes and/or multiverses that have been grouped together in an abstract way. Such a multiverse can be considered like a set of universes which fulfill some criteria, such as "all universes in which Voldernauts are present."
A physical multiverse is a group of universes and/or multiverses that are defined by being physically contiguous.
Multiversal clusters are meant to explain why many universes will share common traits, such as having an Earth, containing humans, or containing the English language, while existing close to each other. Without the concept of multiversal clusters, exoverses where all the universes contain some version of Earth would make less sense.
An exoverse allows you to physically define how your setting works on a level above universal. It is essentially the setting for your "space between universes".