The Action Track is a general-purpose track for clips that aren’t really bound to any specific track. Anything that isn’t directly related to the other tracks can be found within the Action Track.
Depending on whether you added an Action Track within the Director Group or an Actor Group, there will be different clips available to be added. Action Tracks added in the Director Group will have clips that affect global stuff, like setting the environment’s fog color and density, or instantiating objects, while Action Tracks added in an Actor Group, will have clips that affect things on the actor, like making it follow a path, enabling/disabling it, animating blend shape expressions and more.
Once a clip has been added to an Action Track, the track will be locked to accept only clips found under the same category as the first one added.