[Mike McCarty]

Richard,

I believe that it is in place of the white paper. My understanding is that the 18% gray card is the precise "benchmark" from which the camera can measure reflected light.

I don't have any experience with using the white paper method. I would think that one trouble would be that there are all sorts of whites.

My camera does a pretty good job with auto wb if I'm moving around a lot and the card is not practical, but if the scene remains constant I would rather make a quick gray card measurement.

That pretty much exhausts my knowing ...