I'm not totally sure either as I have not heard of this before either. Basically, he took a reading with the L-758 light meter in spot mode of a cloud. He then said.....I know that I can reduce my exposure by 2 1/2 stops to preserve the highlights because I know this camera has a tonal range of 5 stops.
Does that help????
I would like to know this indo for the D3S so I can use it in the same way when shooting landscapes, and spot metering on the brightest area. I can drop my exposure by XX stops to preserve the highlights and still get as much dynamic range out of the image as possible.