This shader can be used either as an environment shader or a texture shader, because it looks up based on the direction of the surface normal. It will map the normal vector direction to a point on the gray ball and retrieve its color.
In the visual effects industry it is common practice to photograph a chrome ball (also known as a “light probe”) on set, as well as a gray ball for lighting reference.
Ideally, one shoots these at multiple exposures and uses software such as Photosphere (Macintosh) or HDRShop (PC) to combine these into a single high-dynamic-range image and/or unwrap the chrome/gray ball into a spherical environment map.
However, it is often difficult to regain the proper orientation of spherical map so it matches the camera used to render the CG scene. Furthermore, a single photo of a chrome/gray ball contains poor data for certain angles that one might want to avoid seeing in the final render.
These shaders are intended to simplify a special case: When the chrome/gray ball is already shot from the exact camera angle from which the final image is to be rendered.
It simply utilizes the mental ray camera coordinate space and applies the chrome/gray ball in this space, hence the orientation of the reflections will always “stick” to the rendering camera.
For additional information and illustrations, see Help menu 
Additional Help mr Production Shader Library Chapter 6: Mirror/Gray Ball Shaders.
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