Since this is a positive displacement instead of a centrifugal pump, the flow cannot be severely restricted or dead-headed. Doing so will result in very high pressure and will damage the pump. This is why the pump curve looks different than a normal centrifugal pump curve and begins at 45 gpm instead of 0 gpm.
The maximum pressure allowed inside the pump case is 22 psi - this is equivalent to about 50 ft head and at this pressure, the pump delivers about 45 gallons per minute.
In other words, this is the minimum allowable flow rate with the throttle lever in the fast position. Remember, if there's no hose on the discharge port, the pump pressure will be near zero and the discharge will be about
80 gpm. But as the discharge head increases (by increasing hose length, reducing hose diameter, or increasing static discharge head), the pressure inside the pump will increase; the maximum allowed is 22 psi (50 ft head) and the discharge volume would be about 45 gpm at this pressure. Of course, the user can reduce to 40 gpm or less by reducing the moving the throttle lever to a slower engine speed.
If you have additional questions, please let me know.