Planes of Existence
The universe as a whole is divided into multiple planes of existence. From a paranormal or supernatural perspective these planes represent different layers of existence and enlightenment (or damnation) where demons, ghosts, and other nasty things come from. Not only that, the planes also contain the realms of dreams, death, heaven, and hell. However, from a scientific perspective they are different layers in subspace diverging farther from the reality everyone knows and they are no less magical or supernatural than anything else; though many plans have distinct and strange properties which call the conventions of reality into question.
This is a matter of perspective. Both definitions are true at the same time just depending on how a person looks at them.
The material plane, also sometimes called the prime material plane, is the focus and center for humans and other mortal beings. This is where reality as everyone knows it exists from atoms to molecules to planets, galaxies, or anything else of that nature. From a perspective of settings most of the RP in all 3 settings takes place. From the inner reaches of the subatomic to the expansive reaches of the universe it is all contained within the material plane. There are many planes adjacent to the material plane such as Hyperspace, The Void, or Lantean Hyperspace among many others. These sit close enough to the material plane as to allow direct interaction with relative ease to the material plane, but they also sit close to others.
It is a common misconception that when a person dies they are either good or evil. This is not the case, in fact they are both. Every person has both good and evil, it is what the person choses to hold onto which determines where they go when they pass on. Do they hold onto love, happiness, and the goodness they have experienced and done in life? If so, they will pass into the Infinite Sanctuary. On the other hand if the person dies and cannot let go of their guilt, hate, anger, and damage they have caused of experienced in life they will instead go into the Severed Realm. In either case, a transformation occurs and when that is complete their remaining energy passes onto the realm it belongs in creating a being of pure purpose.
When a being passes on and cannot decide or is otherwise confused, they will pass instead into the Ocean of Dreams while they put the pieces together of their life and can choose a path to either let go of the horrible hurtful things of life and embrace the good and healing things of life or the opposite. In some cases, however, a being that has died does not know how to embrace or let go of either, continuing to hold onto both in equal measure. When this happens they become a ghost traveling over the dreams of the living and occasionally manifesting again in the material plane looking for some kind of closure or way to let go the mortal world in some way to pass on.
Adjacent to the material plane, Hyperspace is a unique place where the energy of the material world shifts. It is useful for transit because space itself compresses around a vessel and thus allows a vessel to travel at extremely high speeds, relative to the material plane. Defined scientifically as a layer of subspace where the physical laws benefit travel from one point in normal space to another, that is not the whole story. Hyperspace sits between the material plane, the Ocean of Dreams, and the Severed Realm. It can be a very dark place and dark entities are drawn to it, though it is close enough to the material plane to leave it difficult for such entities to interact with mortals or the constructs of mortals.
Very similar to Hyperspace but existing closer to the Infinite Sanctuary than the Severed Realm. It tends to have more encounters with ghosts and other beings which observe or even in rare times advise. Both of these things, however, are exceedingly rare as it is close enough to the material plane to make it difficult to entities from either the Ocean of Dreams or the Infinite Sanctuary to interact directly with mortals.
Star Wars Hyperspace
Similar to both other kinds of Hyperspace, however, it sits close to neither the Severed Realm nor the Infinite Sanctuary. This gives it unique properties that it is actually farther away from the material plane than the other types of Hyperspace and thus allows even faster travel relative to the material plane.
Similar to Hyperspace in that it is an adjacent layer of subspace to the material plane, this layer is a transitionary layer between the material plane, Hyperspace, and The Void. This space is literally defined by its absence of anything else. Since it is a layer in mathematics only matter and energy have difficulty existing here. The Dimensional Fold Drive makes use of folded space to travel through time and space.
Realms of the Dead
One of the most sought after and long questioned planes of existence is commonly considered to be the realm of the dead, where the souls of the departed go once they die. This isn't an entirely accurate representation, however. Souls or essence of a person are embodiments of Aether one of the Five Elements and thus exist separate from the physical body. As such this essence can really go to any plane of existence, though travel through some planes is more difficult and offers more danger to a soul or ghost than others.
This is a complex plane where thought and reality mix in surprising ways. It is a transitional plane where the living and dead can co-mingle but things in this realm are not as they seem. A mortal would have difficulty telling what is real from what is thought here, contact with the dead (especially long dead) is not certain and while energies pass through there, reside here, and generally go back and forth this place is highly complex.
While Christian mythos tends to divide the dead into heaven and hell and this isn't very accurate, this realm is as close to hell as it gets. The realm here is where the discarded souls go after they die when they cannot rid themselves of the negative emotions that have bound them to the material plane. Since they cannot let go of these negative emotions, the energy of the soul becomes corrupted and transforms slowly over time into what one would commonly call a Demon. It is not impossible for a soul to be saved before it becomes a demon, but once the transformation is complete and the soul has embraced the anger and hate that brought it here, there is no going back.
Existing between the layers of reality that boarder the material plane and the Severed Realm, this plane is a rare place which still allows the physical beings such as humans and other creatures to exist and live while drawing its energy from the Severed Realm. The combination of things makes this plane especially dangerous as it slowly corrupts its inhabitants but is also a key place to draw upon the dark energy of the Severed Realm without having to actually go there.
Since there is no true hell there is also no true heaven. This plane is, however, the closest that reality comes to such a place as heaven. When a soul chooses to let go of the anger, hate, and hurt of the world it passes here. The processes of letting go of things of the material plane is a difficult one, here a soul is safe and allowed to explore its life and the lives of others to gain meaning and enlightenment. Once the soul lets go of all the negative aspects of the world it tends to take on a cause of enlightenment, seeking understanding in some deeper way than mortals are capable of.
Each of the Five Elements has a plane in which the energy of that element constructs the interior of the plane, as such there is an elemental plane for Fire, Earth, Wind, Water, and Aether. Each of these planes is close to the material plane in the scheme of things but far enough away to make transition into such a plane difficult for mortal beings.
Plane of Aether
This could be termed as a kind of plane of life or plane of magic, plane of energy, plane of the mind. It is close to the Ocean of Dreams in the organization of the universe and is most commonly called the Astral Plane since it is made of the energy of life a person can enter this plane in their mind and project themselves into other places where life exists as an echo or psuedo-ghost. When this occurs the person projecting themselves leaves their physical body behind but remains tethered to it. If their body dies or their tether is cut, the person's soul effectively is released into the Ocean of Dreams to decide it's fate. However, in the soul dies or is hurt the body will suffer the same fate and harm done to the soul's energy is reflected onto the body.
This plane sits directly adjacent to the Plane of Aether / Astral Plane. The constant churning and swirling Aether of this plane pulls the currents in the Astral Plane most directly but also affect the Aether and its flow in all other realms, the entropy of this plane bleeds through existence touching all other planes. It could even be said that the power here is the core of the changing and shifting nature of life itself.
A shadow of the real world loosely parallel to the prime material plane but separated from it through powerful wards on either side. The plane is fabled to contain much of the darkness and demonic power that once ran free within the material planes, denizens of this chaotic darkness are trapped within its confines by the powerful wards, travel to this plane is dangerous as the plane itself is made up of the darker and more chaotic properties of life. It could even be said that this plane itself is death, or rather the antithesis of life and sanity. The chaos of this plane will drive mortal souls mad if they survive at all. Even if the plane itself does not destroy a visitor, those who dwell there certainly would.
This plane directly touches the Material Plane, Plane of Aether, and the Planes of the Dead making it difficult to creatures to invade the other planes from the Severed Realm. Legend says this plane is here to block the evil from the Severed Realm from invading the surrounding planes. There are known ways to pass into the other planes and not get trapped here, but it is difficult, even for the most powerful demons. One known method is to use a form of Demonic or Unholy Runes to create an anchor and pull part of one plane closer to the Dark Aether or the Severed Realm.
Pockets of Reality
Though not truly planes of existence there are also pockets of reality which act as finite bubbles of the material plane. The main difference is that the planes are of curved space of finite space relative to the plane they exist in. Also sometimes called demiplanes. Inside any curved plane, they are infinite, defined only by the magic which constructed them. They exist attached or anchored to the material plane. These areas include Void Space and the Arcane Sanctuary, in the latter example the space is finite but folds back in on itself to give the illusion of infinite space.
In most cases a constructed plane such as these will have a finite structure or construct within the plane with defined area, though the plane is infinite itself magical or natural barriers will prevent escaping the confines of the defined space. There are exceptions, such as the Arcane Sanctuary which sprawls infinitely throughout its plane. Similar to this is the Firmamentum Saeculorum fits an infinite plane within the structure of a finite castle within Void Space.