Richard,
Manufacturer's of pumps make different types of pumps for different applications.
With most every centrifugal pump, it all comes down to the design of the impeller and it's surrounding housing.
If you tear down an inexpensive powerhead or return pump, you'll see the impeller is just rotating around inside a circular flat housing. It spins and creates pressure within this circular area and drives the water out the discharge. The water moves out because it there is a difference of pressure between the internal chamber where the impeller is, and the atmospheric pressure outside the pump.
As anthem has indicated many times before, these pumps serve us well as powerheads, or in a case where you do not have much head pressure ( backpressure ) working against the discharge of the pump.
They are cheaper to make and work well for powerheads in the tank.
But they are limited to how much they can flow if there's a much backpressure.
The design just is not efficient with backpressure present.
So manufacturer's also make centrifugal pumps that have a different design.
Instead of an small "open" circular area that the impeller spins within, it's a much larger area in comparison, and they enclose this area a lot more. On the inner walls of this enclosed area you'll see "vanes" or raised areas that help channel water. As impeller spins, the water accelerates along these vanes/channels and moves with a greater force. Again there is pressure and out the water goes.
But because of the "better" design, with this enclosed casing surrounding the impeller vanes, the pump is much more efficient, normally moves a lot more water, and most important can handle a heck of a lot more backpressure.
It's not a postive displacement pump, but as the impeller spins, the water exits the pump with greater pressure in relation to atmospheric.
These pump naturally are more expensive.
You can use these types of pumps for most any application, especially on tanks with higher head pressures, or as we're on the topic .... as a "pressure" pump.
No pump likes to be dead-headed - as if you totally shut off the flow from the discharge with a valve. This creates so much backpressure the pump will normally heat up and fail.
Cheap pumps - cheaper design - better for low head, where you want "flow" and have relatively low backpressure.
Expensive pumps - better more efficient design - better for relatively higher head, relatively higher backpressure, and installations that you want "pressure" and "flow".
