For a centrifugal pump you need to read the fine print.
It is sometimes (more commonly) Differential, and sometimes Discharge at a specified inlet condition.
The latter is more common with submersibles where the curve may be for discharge at a minimum submergence.
Max submergence is the maximum depth that a submersible pump should be installed. I would expect that is the maximum pressure that the pumps seals can handle before starting to leak inside the motor housing.
The submergence of a pump or inlet below water level does not reflect in head calculation other than the friction loss thru' the pipe / hose. As bimr has pointed out, the max submergence given is to ssfe gaurd the motor/pump sealing.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)