Ahh, very good question.
Of course, to be fair, above, below and beside are rather vague and imprecise terms when it comes to planes.
In fact, that answer which, I think was me talking, doesn’t makes a lot of sense in reference to what those appendix things say.
However, it actually makes perfect sense to me.
As to what the meat in the Astlan-Abyss sandwich is….
Well–it’s a demizone of smoke, steam, dust and mud. An elemental transitional zone.
[SPOILER]And the individuals from all three planes start intermixing in volume II[/SPOILER]