Final answer:
The question seems to be about operational limits of divers per 'diver down' flag, but it pertains to the improvements in diving operations that have increased diver efficiency and reduced CS exposure, raising the area coverage from 4 m² to 20 m² per dive for experienced divers.
Step-by-step explanation:
The question regarding the number of divers per 'diver down' flag does not specify a universal limit but rather refers to the capacity or productivity increase in the context of diving operations due to innovations. Such improvements have enhanced diver efficiency and safety by reducing exposure to contaminated sediments (CS) and increasing the area a diver can cover. Previously, an experienced diver's carrying capacity was 4 m² per dive, but with these innovations, it has increased to 20 m² per dive, which is a fivefold increase. This data points to improved operational metrics rather than specifying how many divers can be under a single diver down flag, which can be variably regulated depending on local laws or the specifics of a diving operation.