SSS: Emperor
SS: Admiral
S: Admiral Candidates/ Yonko Commander/Warlords
A: Vice Admiral
B: Rear Admirals and Commodores
Below these levels no strict classification.
Not all Stats represent combat power, it's not Doriki but if you have reached such a rating in an ability you have reached that level in an ability.
For example
Nami: Navigation (A+) in East Blue Saga, but as far the panel goes this isn't Combat power even though you could kill an enemy by dragging them to a stormy area.
Future Sight is SSS so is Internal Destruction and Haoshoku Coating.