select an Sphere, activate the move tool, change tool setting to Camera Based Selection. Then go into...lets say...a top view. Marquee drag around all the vertices. If you go back into a side view..you'll see that only the top half of the vertices were selected. Because the camera from a top view could not see the vertices on the lower half.

edit: Also, if you are in a perspective view, marquee drag around all the vertices, then rotate your camera. You'll see that the vertices on the other side were not selected.