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When all conditions are normal, they are closely related..BUT, if you have a restriction [ like a partially blocked filter] , the pumps can have the pressure capacity and the sytem can still have low volume b/c volume flow is a TIME measure.
A partial restriction will still show good pressure at a fuel rail test port, but how long did it take to get to pressure ?? that is where the flow volume comes into play.
So, a fuel pressure test checks both FP Regulated, FP Unregulated, and fuel volume flow RATE. There is also a HOLD pressure time test after you test the FP..that is how well the lines hold pressure after the pumps are off..that test the check valves or accumulators , whichever system is used on that particular chassis.
Flow/Volume measures are particularly important at high demand/load conditions.