He might have been thinking angle of incidence: the prop's pitch angle relative to the hub face. The propeller's pitch angles along the blade may or may not be consistent. In other words, there may be higher or lower pitches at some radii to obtain better performance for certain airplanes. Such a prop, then, wouldn't have any definite "best speed" where there was no slip anywhere along the blade. Some considerations prop maker take into account include engine cooling, where higher pitches might be needed near the root, or blunt, draggy cowlings that might be better off with more pitch outboard to reduce drag on the airplane. The Cessna 152, for example, has alternate props certified for the airframe that improve performance this way.
Dan