Active Surface of the Magnet

The magnet emits flux along its entire surface area.  However, only some part of the flux will be used for the intended application, say to develop force inside a motor or to lift the load in the case of a crane.  

This is known as the active flux.  The Active surface of a magnet is the surface which emits the active flux.

The area of the magnet from which this flux emanates is called the active surface of a magnet.