This video shows how to create a vector based pose space deformer (PSD) in 3ds Max using a float script controller.

A vector based pose space deformer allows you to return a value based on an angle which is calculated by the world space position of three objects.

Pose space deformers are commonly used to drive morph targets and bones. The PSD method for driving morph targets and bones has the advantage of returning a value which is independent of any orientation or axis order unlike the more traditional, rotation driven corrective morph and corrective bone methods which can have problems with gimbal locking and unexpected flipping.