The planes of existence are different realities with interwoven connections. Except for rare linking points, each plane is effectively its own universe with its own natural laws. The strength of the walls between different planes is different in different locations.
But in general the whole world of Myaasia is a place with weak walls, making Magic, transplanar travel and summoning in many instances too easy. Most magic is the application of Elemental planes or emotion planes.
There are two man categories of planes: Full planes and Demi planes.
Full planes are huge, either limitless or with limits so large they do not matter.
Demi planes are small planar fragments varying in size from few yards across as smallest to a normal of few hundred yards to the largest that are few thousand miles.
The planes break down into a number of general types as given below
These support life in much the same way as is familiar, utilizing solar light, food, water etc. But many have striking differences in the flora and fauna present, the existence and power of magic present, natural laws and many other things.
What makes a plane a prime is the fact that the six building blocks are all present and mostly in balance. Further the corresponding spirit planes are in reasonably balanced between the different emotions.
Semi elemental planes
These are planes with abundance of some element and lack of the opposing but not full dominance. The beings from primes can survive most for short periods and with proper protections extended stays are possible.
These planes are the source of mages power and each has one of the six natural forces that make up the primes. Thus: plane of earth, fire, air, water, life, unlife. No beings from Primes can survive here, except with help of protecting spells, but beings from these plane can usually survive on primes, at least a short time since their element is part of the prime mix.
These are the planes "Superimposed" on the Primes and thus quite close, so strong spirits can reach over. There exists one for each prime.
.... There is more of this story ...