Can someone please explain to me the difference between non-dimensional space and extra-dimensional space in DnD 3.5?
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The Terms Extradimensional Space and Nondimensional Space Are Interchangeable
This becomes clear when looking at how the terms are used in context. The most useful example is the portable hole (DMG 264) (20,000 gp; 0 lbs.), which says that it
Emphasis mine. Thus, even within the portable hole's description, the terms extradimensional space and nondimensional space are used interchangeably. That's because such spaces are--no matter which term is used--technically demiplanes:
Therefore the extradimensional spaces of the bag of devouring (DMG 274) (n/a gp; 0 lbs.), the mirror of life-trapping (DMG 262) (2000,000 gp; 50 lbs.), the 2nd-level Sor/Wiz spell rope trick [trans] (PH 273), and the 7th-level Sor/Wiz spell Mordenkainen's magnificent mansion [conj] (PH 256) are the same kinds of spaces as the nondimensional spaces of the bag of holding (DMG 248) (2,500+ gp; 15+ lbs.), the pocket paradise created by the rod of security (DMG 236) (61,000 gp; 5 lbs.), and the quiver of Ehlonna (DMG 265) (1,800 gp; 2 lbs.).
I did some research on this for you.
There are numerous discussions about this around the internet on different forums. I have linked the most relevant here:
What are they saying?
Pretty much that:
Wizards has something to say about it.
There is also an post on the Wizards site discussing it. This Wizards post has a very telling sentence:
It goes on to explain a bit more about these items and some more.
What can we learn from all this?
These terms are vague at best. There seems to be no difference between the two terms mechanically. A Bag of Holding is sometimes referred to as being non-dimensional as well as extra-dimensional - this leads to even more confusion. I think you should just make your own judgement call on what the difference is. But it seems there is none.