Void Beyond

The 'Void Beyond' (時空の狭間 [jikuu no hazama] or 'spacetime rift' in Japanese) is better thought of as 'Spacetime Rift' in regards to the wider Final Fantasy series. The name 'Void Beyond' suggests that it has something to do with the Void, when in fact it is closer in nature to the Interdimensional Rift, and has nothing to do with the Void.

Final Fantasy XIII

The Void Beyond forms the membrane between the Seen World (the physical world) and the Unseen World that forms the rest of existence - as opposed to forming the membrane between the dimensions of the real worlds/universes and the Void, as the Interdimensional Rift does.

Quite according to its (non-mischaracterized) name, time does not exist in the Spacetime Rift/'Void Beyond', and a person could stay there dreaming forever if they once fell asleep. It is also possible for things to become real in the Spacetime Rift if enough people wish for something for a long enough time, as is the case with Serendipity. People (and the souls of the dead) who fall between cracks in time end up in the Spacetime Rift, and one of the destinations they end up in is Serendipity. Another name for the Spacetime Rift is 'Shadow of Valhalla', as reflections of Valhalla fill up certain parts of this dimension.

The Void Beyond should not be confused with Historia Crux in the FF13 universe, which forms the membrane between the real world and the Spacetime Rift/'Void Beyond' and allows time travel to happen. It is in fact not in the Void Beyond, although is in the Unseen Realm. There is also the concept of dimensional distortions, but it bears repeating that none have anything to do with the Void as the concept is in the wider Final Fantasy universe.

Dimensions II

The Rift (時の狭間 [toki no hazama] or 'space between time' in Japanese) is a space between time and the physical world. Note that it is subtly different from the dimension present in the FF13 series, although is listed here due to their extreme similarity. It is not the Interdimensional Rift either however, as that is explicitly called a space between dimensions, where this Rift is a space between time - rather more like Historia Crux of the FF13 universe.

No time passes in the Rift, although sometimes fragments of the physical world are reflected there, or even transported there. It is usually inaccessible to humans, although moogles can traverse it freely, and the Esper Atomos has access to it as well. It is unknown if any other Espers exhibit this ability.