The PLAVelocity Tag together with the GlobalPSR Tag (also included) are tools to do velocity based interaction. A Softbody is a very good example. The points of a Softbody have different velocities when the body is deformed. The PLAVelocity Tag needs a PLA animation to be able to efficiently calculate the points current velocities. The velocity values will be written to a Vertex Map.
The Vertex Map can be used to drive a Shader or restrict Deformers for example. Many things are possible with Vertex Maps.