The oil pump is directly connected to the camshaft so pumped volume is a direct function of engine speed, not pressure.

A high volume pump has larger rotors so for each revolution it pushes a larger volume of oil into the engine.

A high pressure pump is a pump that has been equipped with a higher load relief valve spring. You can put the high load spring in either a standard volume pump or a high volume pump.

People are always getting confused on this topic, most likely because they forget about how the relief spring works.

If the relief spring is closed then a higher volume pump will pump more volume into the engine (and raise pressure). If the relief spring is open then the high volume pump basically does nothing but recirculate more oil inside the pump.

The big difference between a high volume pump and a high pressure pump will occur at low engine speeds. The high volume pump will produce higher pressure at idle since it is forcing more oil into the engine. At anything greater than about 3000 rpm the high volume and standard volume pumps start to act the same since the relief spring is blowing off all of the excess volume.

Sometimes the high volume pump will overwhelm the relief port and pressure will continue to increase past the relief point. Put a Milodon pump on an engine with tight bearing clearances and you'll see that happen. At 6000+ rpm the Milodon pump is putting out so much volume that the relief port can't keep up and the oil pressure will go thru the roof. That combination is a good way to blow the oil filter apart. Bad thing is it will happen when you're going 140 mph thru the lights.

From what I've seen over the years, there is very rarely a need for a high volume pump on a big block Mopar. Maybe if clearances are on the high side and full groove mains are used but otherwise it is just a waste of energy.