When folks say Planescape is traversing the Astral Planes, what does that mean in practice? Because it sounds like it means detailed preparation for an amusingly huge collection of possibilities
That's Spelljammer, you travel the astral (5e) /ethereal (3e) /phlogiston (2e) in ships to other worlds outside the multiverse (but not always). In Planescape you usually travel directly via portals, rifts, spells, or transient planes like Sigil and the outlands to other planes within the multiverse. EDIT: Also the infinite staircase.
Both can offer a huge amount of story possibilities, because of the sheer scope. Notably, 2e Spelljammer had a map of the stable paths that allowed one to travel between all the known settings of DnD, it included Forgotten Realms, Greyhawk, Krynn, and others.
