In reference to Dark Sun and Spelljammer, I remember reading a specific section in one of the official Sourcebooks which covered how the Phlogiston allows access to the various AD&D Settings. It goes like this:
1). All Systems are enclosed in an impenetrable Crystal Sphere or Shell, which keeps the Phlogiston out. Spelljamming allows one in and out.
2). The Crystal Sphere is marked [on the inside] with the Stars, etc. of the local World Setting, and contains all of that Setting’s Stars, Planets, Moons, Comets, etc.
3). The Crystal Sphere is usually “Open”. This means that Divine Access [for Divine Magic, etc.] works, and that the Outer and Astral Planes can be accessed. It also means that Spelljamming, and Arcane Spellcasting work normally, as per their standard rules.
4). The Athasian Crystal Sphere, from what I have read, is a Closed Sphere. And it always has been. Thus, no Divine Access, no access to the Phlogiston for Spelljamming, no replenishment of Arcane Energy, and in general, an increasingly post-apocalyptic Setting where the despotism of The Sorceror-Kings reigns supreme, and desperation in life and struggle for survival are the order of the day.
5). Athasian “Divine Classes” such as the Athasian Cleric, or Athasian Druid, are gaining their Spells from the Land, and the Elemental and Natural Guardians thereof, rather than an external Planar source, like standard Clerics; The Templars of the various City-States gain the power directly from their local Sorceror King/Queen instead.
Hope all of this helps as an [unofficial] clarification.