This shader continues a ray of a given color and merges the result with an input color. The purpose is to continue a ray as if the current intersection did not exist. Trace depth, ray type, and distance for volume computations are not modified. A typical use is for walls of a showroom where the camera sits outside, so that the room walls must be ignored.
For information on this shader, see the mental ray Documentation at www.autodesk.com/mentalray-help-2013-enu.