The text of that Extraplanar subtype, from the SRD:
This subtype is applied to any creature when it is on a plane other than its native plane. A creature that travels the planes can gain or lose this subtype as it goes from plane to plane. Monster entries assume that encounters with creatures take place on the Material Plane, and every creature whose native plane is not the Material Plane has the extraplanar subtype (but would not have it when on its home plane). Every extraplanar creature in this book has a home plane mentioned in its description. Creatures not labeled as extraplanar are natives of the Material Plane, and they gain the extraplanar subtype if they leave the Material Plane. No creature has the extraplanar subtype when it is on a transitive plane, such as the Astral Plane, the Ethereal Plane, or the Plane of Shadow.
The Banishment spell doesn't say where they go, just:
A banishment spell is a more powerful version of the dismissal spell. It enables you to force extraplanar creatures out of your home plane. As many as 2 Hit Dice of creatures per caster level can be banished.
This spell forces an extraplanar creature back to its proper plane if it fails a Will save. If the spell is successful, the creature is instantly whisked away, but there is a 20% chance of actually sending the subject to a plane other than its own.
So this begs two questions.
- Does Banishment also carry the 20% chance to go somewhere random? (Banished from Plane of Air to... Plane of Fire!)
- If the PC is sent back to the Prime Material successfully, Do they "automagically" go to their own world? To their own continent? To a anywhere they have ever even known?
In other words, if some meddlesome PCs from Forgotten Realms (say, as an example) muck about in the planes, could they get banished to somewhere like Kara-Tur, or Maztica? Or could they even find themselves further out from "home;" stuck in the world of Dark Sun or Greyhawk?