Planejammer: Ad Astra Per Arcana
In the Macroverse , there are many dimensions, of which the Planescape/Spelljammer Multiverse is only one. These dimensions can be pictured as floating through the Far Realm in much the same way that Crystal Spheres float through the Phlogiston o the Prime Material Plane of our Multiverse
The laws of reality can vary wildly from one dimension to another. For example, there could be several dimensions the laws of physics and magic work like the ones in the Planescape Multiverse, with only slight differences, while others could be quite similar to our own. Our Earth (d20 Modern) and its universe are one dimension, and there are many others that “mirror” our earth such as the so called Gamma Terra and Dragonstar dimensions.
Whether there is a finite or infinite number of dimensions is unknown (like layers of the Abyss), but there are certainly a multitude of them, making for a dazzling array of possibilities.
Dimensional travel is extremely difficult except under certain circumstances, and few know of the other dimensions, much less understand them.
Elminster of Shadowdale, Chosen of Mystra in Realmspace, has documented many trips to our own reality over the years. Murlynd of Greyspace spends much time in a world that resembles our own past in the days of the Old West. It is not unknown, although the knowledge is rare.