That's the real question, isn't it? The Void is of an umbrella term for Eorzeans, used to describe anything beyond conventional wisdom's worldview. For that reason, it's easy for us as observers to attribute everything we don't understand to the same ideal. What if Louisoix's time-limbo is the Void? What if the Seven Hells are of the Void? What if reality is contained inside a bubble of concentrated aetheric energy that forms reality and everything outside of it is the Void? Is it all of those things?

In FFI and FFVIII we see that the Void can be used to traverse time, which makes as much sense as one can hope for. If reality is aether and outside of it is Void, inside of it is where physics matter and thus is where time exists. Likewise, a common theme is that it is separate from physical reality (both the light and dark worlds, if indeed both are used), and can return it to the nothingness it once was if the crystal were to be destroyed.

I should clarify that this isn't building up to a point, I'm just musing again. However, after seeing what very well may have been a dark crystal, perhaps the Seven Hells are actually the Dark World and we have yet to see the true power of Void (outside of perhaps Atomos?), but then again, Atomos spawned the very same demonic entities that the Ascians do, so perhaps we've cast aside the dual world method for FFXIV and we're just seeing an abyss of intangible ideas and essences that are made manifest by concentrated aether in one world of light under the crystal.

If I were to create an alternative theory to that of Catapult, mine would state that every world in the Final Fantasy universe (aside from VII and X) takes place in a different reality, a different canon, and a different world, but many share the same Void.

Think about it this way: What if the big bang isn't the start of the entire universe, but simply our bubble of it? The solar and galactic levels weren't the top of the chain, might there be vast distances of void between universes that share different make-ups and laws? Perhaps the Mothercrystal of Eorzea is but one of many aetheric concentrations in the void. Sure, she makes up all of our reality, but across the Void another concentration forms another Mothercrystal and within that isolated bubble of reality exists Vana'diel.

What I really mean to say is that I have nothing to offer you aside from that bit of lore above. We could see them do anything in the future.