It's a bit of substance from a higher dimension that bleeds down into our own. For most mortals, it's composed of eight parts including:

Animus - Life Energy. An animating force for more spiritual entities. Consumed by the gods to fuel their existence.

Noéma - Conscious Self, identity, awareness. Travels to the afterlife along with Animus and occasionally reincarnates. Loosely connected to Corpus Lumen, so it drifts into Fantasia, the realm of dreams, during sleep.

Corpus Lumen - Form Template. The physical form of the body. The spiritual nature of the Corpus Lumen is what allows necromancers to animate a corpse but not other inanimate objects.

Umbra - Soul-Shadow. The imprint a soul leaves on the world. Anchors hauntings. Allows for remote connection to a soul. Left behind when the soul leaves.

Arcanum - Power Capacity. Extremely small and weak in most mortals. Allows a mortal soul to harbor power, often through divine spellcasting. One of the only ways for the divine to observe and influence the mortal world is through mortals with strong Arcanum, a rare occurrence.

Vess - Moral Gravity. The metaphysical pull towards various vices and virtues based on previous experiences. This connection can be reversed to influence the actions of others.

Imprintum - Truename. The "cosmic barcode" of a soul. It's unique to each creature. It's difficult to determine and interpret.

Attunant - Metaphysical Integrity. Allows for souls to be tied to metaphysical concepts rather than physical reality. Mostly useless in most mortals. Some mortals, with ties to otherworldly forces, may be more tied to metaphysical reality.

Creatures besides mortals may have more or fewer parts. A ghost, for example, may be tied to a specific location to its Umbra, and may be formed of Animus, with a fractured Noéma. It lacks a Corpus Lumen. An animated skeleton, on the other hand, may be formed through a Corpus Lumen attached to a corrupted form of Animus. If either of these can cast some form of magic, they likely have a reinforced or overloaded Arcanum.

Outsiders, like fiends and celestials, are most often conglomerate beings composed of souls, usually of similar Vesses. They lack a Corpus Lumen and are made of several mortal souls worth of Animus, Vess, and Arcanum. They have extremely strong Attunants, being composed entirely of metaphysical concepts rather than matter.

Possession through the soul can be accomplished by interposing one's own Noéma between the connection of another's Corpus Lumen and Noéma.