The Tool Possession Phenomenon is a reasonably well-understood phenomenon, where an inanimate object is possessed by a Spirit. This happens as a result of a Spirit's innate desire to interact with the Material Plane in conjuncture with their ability to slightly manipulate random events.
In essence, a Spirit has the ability to change the outcome of nondeterministic events. However, this is not as simple as desiring a specific outcome in order to make something happen. Rather, it can be compared to the process of composing music: There is a large but finite amount of possibilities for input, but only very few combinations will create an output that is distinguishable from random chance, and even fewer will have the desired output. Even so, if one is willing to put enough time and effort into it, the desired result will eventually be reached.
That means that in order for a Spirit to effectively possess an object, it first needs to spend a certain amount of time familiarizing itself with that object. The amount of time required is directly proportional to the complexity of the object. Hence, Spirits usually only possess objects if they feel a worthwhile payoff is to be expected.
This condition usually holds true for objects that allow for a relatively strong influence on the Material Plane, as well as objects that are often interacted with by entities with a Physical Form. The latter was what first gave rise to the name of this phenomenon, since it was observed that especially cherished tools, weapon and armor had a high chance of being possessed by Spirits. This is because this allows the Spirits to be close to whoever is using the tool, and thus satisfies their desire for interaction and contact.
The other category of objects that Spirits tend to possess are objects which, by their very nature, rely on chance for semi-autonomous behaviour, such as complex Iserial Programs or Golems. Some of these are, in fact, created with the intent of having them be possessed by a Spirit, and as such aim to make it as easy as possible for Spirits to use them.