Three things sit on the substrate beside the coral, each doing one job.
Four reference charts, toward the corners of the frame, labeled A to D clockwise with A toward the recorded bearing. Their known size gives the scale. Their circles let the software work out exactly where the camera was. And because four charts of the same size appear the same size only when the camera is square to their plane, they are also a spirit level you can see.
The white and gray panels on those charts give the exposure and the neutral point. The white panel matters more than it sounds. Bleached tissue is white; a clipped highlight is also white; so a frame shot a stop too bright will manufacture bleaching that was never there, and the resulting photograph will look perfectly fine. The white panel is what catches that.
A coral health reference, wherever live coral is in the frame and one is available. This is what condition classes are read against. Scoring bleaching from impression — that looks pale to me — is not reproducible by a second scorer, or by you a year later on a different monitor. A standard reference lying beside the coral turns an impression into a comparison. It is recommended rather than required; if you did not have one, your Entry says so, and your condition classes are reported as impressions rather than as comparable measurements.