The user can manipulate the soul, the incorporeal and immortal essence of a living thing, souls are usually most obviously present in sentient beings. At some level all biological organisms have souls, and even non-biological entities (such as rivers and mountains) possess something that is analogous to a soul.
Most users have to learn to control their own soul first and expand from there, some may be able to learn how to tap into the essence of everything living.
When judging the potency of Soul Manipulation, and the resistance against it, there is a variety of factors to be considered. Such as the mechanisms involved, whether it has demonstrated to break through resistances, how great the effects are, if it can affect higher-dimensional beings, etc. Numbers of souls affected should only be considered as a form of potency if the verse clearly establishes as such, such as Shinza Bansho.
It should be noted that the ability to interact with souls and other non-corporeal entities directly as if they were physical objects is usually considered Non-Physical Interaction and does not grant the user the ability to manipulate souls in other contexts.
Under normal circumstances, most characters are assumed to have a soul by default unless the original medium specifies otherwise. Those who do not possess soul are usually considered to be immune to most applications of Soul Manipulation.