above "C" rank aswan potentials

i just want to know what/how the game identifies as rank X

Well, if you wanna know what I think, though I haven't put much data together, I'm thinking something along these lines:

Rank	Max Level: 10	Max Level: 20	Max Level: 30	Max Level: 40
C	-	-	-	1-10
B	-	-	1-10	11-20
A	-	1-10	11-20	21-30
S	1-10	11-20	21-30	31-40

Where x-y is the level range available at that rank.