I look at it like this... (for an EA Varipump 20000). A 5.3m max head with a 2" (50.8mm) connector.

Let's say the filter is setup gravity fed, with a pipe back into the pond. With the pump off then the pressure of water through the BD and filter to the return pipe is equal - so we don't have any flow.

If the pump is at the lowest point then it has a higher pressure from the pond balancing the longer return above the pipe.
If the pump is just under the water level, then it still has that same pressure from the DB but the pressure at the pump in should be the same the pressure in the pond at that depth.

Switch the pump on and it is simply adding pressure (through the impeller) to that circuit.

So if the pump is at zero depth and the max head high is 5.3m then it's got X bar of pressure being added before the flow reaches zero. The effect of pressure and pipe length is also related to the amount of water in the pipe (it has weight) so this is what is causing the main back pressure. Increase the size of the pipe diameter and the amount of water and pressure it creates increases too so the max head drops (this is offset slightly as the back pressure caused by friction on a wider diameter pipe drops).

The longer the pipe, the more bends, the wider the diameter, the more back pressure to the pump and the slower the flow. (this is also true on the suction side of the pump - fewer bends, more sweeping wider pipes).

If you believe the paper stats - my new filter is speced at a max of 16500l/h or about 80% of the pump max rate. However add piping and a run of 63mm pipe straight up about a meter above the pond level (current design has a veggie 'stream' that overflows into the pond) then that max pump rate is going to be less.
The new pond will be about 6675l so I can dial back the pump and rate to something more leisurely - probably 8000l/h given there are 5 koi that will be in it.