The Assign Controller rollout assigns and appends different transform controllers to individual objects. You can also assign controllers in Track View.
Animation controllers are plug-ins that handle all of the animation tasks in 3ds Max. For a complete list of available animation controllers, see Assign Controller (Track View).
Example: To assign a TCB Rotation controller:
The default Euler XYZ Rotation controller is replaced with the TCB Rotation controller.
Opens the Assign Controller dialog. Available only when a track is highlighted.
Choose a controller from a list of available controllers in this dialog.
The Assign Controller dialog lists a subset of the different types of controllers whose members depend on the type of track you've highlighted. For example, only Rotation controllers are available for rotation tracks.