Most entities in 3ds Max that the user can see and interact with, including renderable entities such as objects, and non-renderable entities such as controllers, modifiers, lights and cameras, are derived from the Animatable base class. Most plug-ins types are Animatable derived classes.
Animatables have parent-child relationships with each other, to create an animatable hierarchy. Animatables which are children of other animatables are known as sub-animatable, or sub-anims.
Most sub-animatables are also references. If there is a correspondance between the sub-animatable index and reference number, a value other than -1 will be returned from the function Animatable::SubNumToRefNum().
The 3ds Max track view is the visual representation of the animatable/sub-animatable hierarchy.