Curve A corresponds to a system with high head losses. This is typical of a new pool with 2” PVC plumbing.
Curve B corresponds to an older system with very high head losses. This is typical of a pool with 1 ½” plumbing.
Curve C corresponds to a system with medium head losses. This is typical of a new pool with 2 ½” PVC plumbing.
Clearly James and I have different views on what the plumbing curves represent. There is a lot of variance depending on what one assumes. For example, if one assumes a pool/spa setup with single 35' runs, a cartridge filter, NG heater with internal bypass, pool/spa valves, 12x90s, 4x3/4" eyeballs, you would get the following plumbing curves for each pipe size.
1.5" plumbing - Head (ft) = 0.01144 * GPM^2
2.0" plumbing - Head (ft) = 0.00691 * GPM^2
2.5" plumbing - Head (ft) = 0.00580 * GPM^2
If you assume a DE filter with a backwash valve, it gets closer to my definitions.
1.5" plumbing - Head (ft) = 0.01195 * GPM^2 - Between Curve A & C
2.0" plumbing - Head (ft) = 0.00814 * GPM^2 - Close to Curve C
2.5" plumbing - Head (ft) = 0.00719 * GPM^2 - Below Curve C
However, the reason I use Curve A for 1.5" plumbing is because older pools tend to have less eyeballs and small sand filters which greatly increase head loss. With 2x3/4" eyeballs and a 2 sq-ft sand filter , the curve constant becomes 0.01746 which is closer to curve A. In order to get to definitions James presented, it would require a significant increase in head loss somewhere.
BTW, Curve B is nearly impossible except for plumbing systems that have something seriously wrong with them.