I think that is a way of estimating the impacts of fittings on pump output. Most people don't care to know the output exactly, they are aiming for a range, with the lower end greater than zero. A lot of pumps are available that struggle with pressure and the estimates are provided so folks can avoid getting one of those.
New pumps should provide a maximum head number for planning. Some pumps are listed as pressure pumps, the others are flow, again as general ways to decide if they will fit your application. Pressure pumps for going up a story in the house and flow pumps, with flow ratings at head distances typical for short aquarium lifting purposes.
