This effect is the only one that can (and must) be applied to a Camera layer. It allows special tracking setups, e.g. when a camera movement is tracked for a real/virtual TV studio setup. According to the (real) movement of the camera Pandoras Box renders a cut-out of a virtual space which is then displayed as a background in the (real) setup. Therefore it is seen by the (real) camera which results in a recorded mix of real and virtual elements. Only from the real camera's viewpoint the background is in the correct perspective. This setup is an alternative to keying applications with a green or blue screen.
The tracking data changes the parameters of a PB camera layer. The effect parameters on the other hand are static, i.e they are setup once as fixed values. The four coordinates represent the corners of the screen whereon the background is projected and can also be understood as a "window" through which the (PB) camera sees and renders the background.
An example for this effect is the setup of the coolux booth on NAB 2013 seen here: https://www.youtube.com/watch?v=7ectw1Z9BvA