As far as grades go, I find it much easier to understand percentages of curl shown than any grading system.
Type of curl is whole other thing-and as stated by others has some overlapping terms.
Speaking of LPI of the endgrain, we could also use "stripes per inch" or "curl per foot" maybe to designate narrow or wider spacing of "normal" curl in Maple (and similarly striped woods).
Yes I do see more variation in curl in Red Maple, and some of it very attractive, than in Acer Saccarum, but I've not worked with any yet.