A key aspect of The Parallel Planes, and indeed, that which the series is named after, are the planes.
Plane of Radiance
Also called: Radiant Plane
Also called: Primary Plane, Plane of Existence, Material Plane, Real Plane, Natural Plane, Plane of Life, Plane of New Life
Plane of Shadows
Also called: Plane of Darkness, Shadowy Plane, Other Plane
Why so many names?
When Viktor Soria sought to assign precise names to many objects/concepts (different types of mages, distances, etc.), a lot of letters circulated throughout Reyseen, people suggesting different names. Some of them were already in use, some were new. While Plane of Radiance, Physical Plane, and Plane of Shadows are the accepted official names (official as declared by Reyseen), other names are still widely used.
Long ago, there were only two planes: the Plane of Radiance and the Physical Plane. Perhaps even longer ago, at the beginning of time, whenever that was, if there even was such a thing, there was only the Plane of Radiance or the Physical Plane (assuming planes don't die, so to speak... if they did, perhaps countless planes have come and gone), but that knowledge has long been lost.
Talk about how wile didn't naturally exist, just as computers or certain radioactive elements (not Elements) don't naturally exist. All souls went to the Plane of Radiance.
The Plane of Radiance is a place of magic: idyll and ether. It is a place where souls exist (amidst idyll), and souls are the stuff of ether. Some mages suspect ether isn't truly native to the Physical Plane, but rather what ether they use is somehow overflow from the Plane of Radiance, or something of the sort (they're wrong). Perhaps mages have the ability to bridge the planar boundary to a limited degree. People who believe the planes are distinct locations are fans of the former, while people who think the planes overlap in some manner are forms of the latter.
Ether, like many things in life, has its opposite: wile. If ether is addition, wile is subtraction. That isn't to say magic does nothing but create and wile does nothing but destroy, but rather they cancel one another. Focusing our attention on Consen long ago, few Kyaron were born with the ability to use ether. Most used it for what it was, learning how to isolate Elements and utilize them. A few Kyaron, perhaps with some natural inclination, used their ability in another way; they figured out how to produce ether's opposite, “material” called wile. It had radically different properties and means of use, but still had its uses.
Daitya Stamatis incidentally figured out how to make permanent bubbles of wile. That is, while mages utilize the ubiquitous ether, warlocks and witches had to work to produce their wile before they could use it. After a grand ritual, Daitya made a massive bubble of wile, bigger than the entire world. It was a pale reflection of the world, a place of wile. This place is the Plane of Shadows.
A person born in the Physical Plane (beings aren't born anywhere else, for that matter) is born with a pure soul of ether. Actions which corrupt its purity let in wile. A released soul (that is, when a person dies) with too much wile gets drawn to the Plane of Shadows.
Note when comparing the three planes, it's best to start with the Physical Plane, the one we're most familiar with. The Plane of Shadows is just like it, only cold, pale, lifeless. The Plane of Radiance is something else entirely, a “place” which we can't imagine, due to our imagination being limited to our senses.
Compare and Contrast
PoR: Infinite, unbounded. It's like a manifold.
PP: Finite, unbounded.
PoS: Finite, bounded.
PoR: There is a sense of time in the sense that when a person does a task, there's a beginning, the task itself, then an end, but it's not quite like the familiar one.
PP: The familiar one.
PoS: Identical to the familiar one (inherited, perhaps).
PoR: Layered. Bottom layer is the abyss, above that is purgatory, above that is where most people hang out, a paradise for every day “life,” above that is a place of singing and worship, above that is something else. It can sort of be thought of as a pyramid, but the layers aren't finite, and the abyss is more like a little room below the corner of purgatory, so it's not the “biggest” layer.
PP: Not layered.
PoS: Not layered.
PoR: Not so clear. For example, when interacting with someone, you must do something to express your thoughts. Speech, gestures, written language. Not so in the Plane of Radiance. A good way to think of it is with our physical bodies shed, we have fewer differences/limitations.
PP: Different objects are clearly different, and it takes some effort to interact.
PoS: Identical to the Physical Plane's.
PoR: Souls go from the Physical Plane here, good ones at least. And it seems to be the case that this is not limited to just of humans, as the Plane of Radiance has an abundance of life (but nothing procreates). Innocents, like plants and animals, seem to come here as well.
PP: The only plane where life comes to be, at least nowadays.
PoS: Inhabited (almost) exclusively by beasts, which are the souls of wicked humans who did not have a strong enough will to retain their identity while passing through the planar boundary. Those few with enough willpower remained themselves, constituting the few “humans” who live in the Plane of Shadows, excepting the sins and the King of Evil, who are unique. Lastly, there is a very, very small amount of vegetation, its origins unkown.
PoR: Depends on the layer. The one where most “people” reside is idyllic.
PP: Like our own.
PoS: It's always night, lit by an enormous gibbous moon. It's mostly just barren, like a desert but not sandy. There are a few structures (built by the people, of course, perhaps with the aid of what little ether there is), but not many, and a few plants (mostly dead trees, maybe leftover from its creation).
PoR: Idyll. For more info, see the other file.
PP: Ether. Likewise.
PoS: Wile. Likewise.
First, a note on Summoning (Earth+Darkness). The discipline/specialty school called Summoning has four (known) uses, two of which are pulling a being from from the Plane of Shadows to the Physical Plane, and ripping apart the planar boundary. Likewise for the other version of Summoning (Earth+Light).
From PP to PoR: Be a good person and die, or step through a rip in the planar boundary.
From PoR to PP: Essentially impossible. In fact, there are two ways. Do the other version of Summoning to breach the planar boundary, or go from the abyss to the Plane of Shadows, then the Plane of Shadows back to the Physical Plane by means of being Summoned or stepping through a rip.
From PP to PoS: Be a bad person and die. Alternatively, Summoning. The final means is Lita.
From PoS to PP: Step through a rip, or Lita.
From PoR to PoS: Abyss.
From PoS to PoR: None. The abyss is a one-way trip. It might be possible to Lita to do this, but she has to be familiar with the place, and she's not familiar with the Plane of Radiance.