What is an Action Node?

Arguably one of, if not the most popular thing the modding community likes to mess with.

In BULLY, just like many other games, characters (and even props) have a wide array of actions that can be performed.
Something that should be mentioned is that there's a difference between animations and actions.
The only properties that animations have are the playback speed and movement (walking, sprinting, strafing, etc).
On the other hand, actions are animations that can have multiple properties attached to them, such as sfx, vfx, etc.
A raw punch animation alone won't actually hit the opponent or produce noise.

There are files using the .cat extension inside the Act.img archive, which can be speculated that means "Compiled/Compressed Action Tree".
The source version of these files uses the .act extension instead, which stands for "Action Tree".

Action trees are usually used as a way to represent the various possible actions that a ped can perform depending on the situation.
These trees are made of action nodes, each one being a different action that a ped can do.
All trees start from a root node and, from there, it starts to "branch" into more action nodes until they meet an end (just like a real life tree, hence the name).

Action nodes tend to have conditions on them as a way to know if the ped should or should not use that action at the moment.