Grading designation question
I've been looking at a few coins lately and have run into "grading numbers" for lack of a better term, that I can't find what they mean. For example, XF-40, N-8, R-3. Well, XF-40 is easy. But what's N-8 ? I found out that "R" is a Rarity rating and that seems to vary depending on which scale you use and coin type, But I'm stumped for the "N" number. Is there a chart somewhere. Can anyone point me there?