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That PCB is used for alle aquaduct variants. Depending on the aquaduct model some pins have no function or offer to add other stuff as an option.
The aquaduct 720 XT already comes with a built-in flow sensor so you don't have to buy one.
The relay connector allows to shutdown your PC if a specific alarm occurs. The standy by connector must be used if you connect the
ATX break adapter for the mentioned shutdown option, otherwise you would not be able to start your computer because the relay will stay open without the stand by power.