Mostly the latter. Or, for some very short focal length lenses like the 14 & 15 mm kind, probably the combination.
The 15/5.6 QDC, first Nikkor in the 15 mm class, was notoriously difficult to focus even with the otherwise excellent coarse-grained F2 screens. Thus Nikon provided a special screen, type R, for it. It was excellent with the 15 mm and pretty much useless for anything else.