Actor

Actor is the main class in StepMania. Pretty much everything is an Actor, including ActorFrames.

StepMania 3.9
In StepMania 3.9, files with the .actor extension are considered Actors, as well as each layer in a BGAnimation.

Example
This .actor file implements a BitmapText object:

and this BGAnimation file calls the Actor:

StepMania 5
Actors are a lot more prominent in StepMania 5. LoadActor will take care of most Actor types for you.