The geometry for the Mag Zone is real, and you can select it (at least in P6). The Zone uses the same geometry as the ball primitive "ball.obj".

If you need proof, try this. Under the 'Geometries' folder, find the 'props' folder. Rename the "ball.obj" as "ball.BAK". Make a copy of the "box.obj" and rename it as "ball.obj". Now create a new magnet in Poser, the new magnet will have a box shaped geometry for the Zone. Of course this does not change the effective shape of the zone itself, the geometry is only a visual icon to show you where the Zone is, and what shape it has, but the geometry associated with it is real Poser geometry. Same goes for the lights and cameras.

Quote - (I do not know why and who need it)

I would be absolutely lost without the Zone geometry. How else would I know where the Zone is and what shape and orientation it has in the scene? Same goes for the Base, at least until the initial position has been set. The geometry for the Mag on the other hand is fairly useless, and I often hide the Mag geometry before I start using the magnet. I have even considered replacing the default Mag geometry with a null.obj.

Deleting the "buildSphere" attribute will remove the zone geometry, but only temporarily. If you save the scene from poser and reopen it, the zone geometry will be back. Again I must qualify all this by saying this is how it works in P6, later versions may work differently.