When you create an instance of Mathdeform in the scene a small rectangular patch will be create. This plane has been generated by default until you select an object . As you click on the Select Object button it turns to active mode and Max ask you to choose an object. The cursor shows that which objects are acceptable for Mathdeform. Basically all objects in category of geometries would be allowed to pick by Mathdeform. After you select the object the plane will be transform to the same object you picked.

Maximum Length:
It defines the maximum distance that any vertex can get from the pivot point. All vertices locate further than than this will be ignored.

Experssion1 translates all vertices of the box into spherical coordinate system. The result passes to X,Y and Z functions by <myp> as a spherical point variable.

Note : see the getspherical  on extra functions.