A soul is a spirit that has become self-aware. It also has a full expanse of free will upon the matter it affects, and near-infinite free energy potential (although at this level, it's impossible to harvest beyond what the body is capable of). So great are its powers at this stage that it constructs a representation of the matter it affects in the same level of reality as natural forces, and affects those instead of affecting reality directly.

All humans in Ljëne have souls. Animals typically are at the level of spirits, although there are some exceptions (for example the Alphoinje)

This is very similar to the process of signatures, and so a soul can be thought of as a mandala of interacting signatures which converge in something that blends them together into something different in the center, which then can affect the signatures on the outside and protect them from being nullified. This is essentially where the mind comes from, and what it is.

Put more simply, signatures affect the lowest, material level of reality. Spirits are capable of influencing the material level and themselves, to a very small degree. Souls are capable of affecting not only reality but the individual signatures that compose them.

When certain signature types become the dominant ones in a soul, (they are generated by the soul itself), then the soul is capable of affecting other signatures that are not itself, and by proxy, other matter that is not itself. When this happens, a soul becomes Ascended.