A photographic record that can be measured. An ordinary underwater photograph cannot be. Nobody can say how large anything in it is, because there is no scale. Nobody can say what color anything really was, because the water took some of the light and the camera guessed at the rest. Nobody can say where the camera was, how far away, or whether it was tilted.
The method removes those unknowns not by being careful but by putting the answers in the frame. Everything else follows from that.
From those images, two kinds of number. Benthic cover — what proportion of the surface is hard coral, soft coral, macroalgae, turf, rubble, sand — estimated by scattering points over the rectified frame and asking what lies under each one. And condition: bleaching, recent mortality, breakage, recorded separately from cover.
And a written protocol precise enough that somebody who has never spoken to you can find your station and repeat what you did.