The Action Track is a general purpose track for ActionClips 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 ActionClips available to be added. Action Tracks added in the Director Group, will have ActionClips that affect global stuff, like setting the environments fog color and density, or instantiating objects, while Action Tracks added in an Actor Group, will have ActionClips that affect local stuff related to the specific actor, like making it follow a path, enabling/disabling it, or animating it’s character expressions.
Once an ActionClip has been added to an Action Track, the track will be locked to accept only Actions Clips found under the same category as the first one added.