There are a few special rotations that are difficult to arrange manually, quite commonly desired, and are best described as 'internal motion'. The buttons in the box marked Internal Rotation give a quick way of specifying motions that are 'internal,' that is, relative to the Actor, no matter what other actions the Actor is performing. The rotor blades of a moving helicopter are a good example of internal rotation. The Actor's direction can still be specified in its key frames, while the internal rotation is 'underlaid' on the tween.
None - No internal rotation.
Vertical Axis - Actor is rotated internally through the vertical axis (the Z axis in the Top View) of the Actor's center.
Side Axis - Actor is rotated internally through the Side Axis (the X axis in the Side View) of the Actor's center.
Front Axis - Actor is rotated internally through the Front Axis (the Y axis in the Front View) of the Actor's center.
Clockwise - Actor is rotated in a clockwise direction in the specified Vertical, Side, or Front Axis.
Anti Clockwise - Actor is rotated in a counter-clockwise direction in the specified Vertical, Side, or Front Axis.