Tier 1 = G

Tier 2 = F

Tier 3 = E

Tier 4 = D

Tier 5 = C

Tier 6 = B

Tier 7 = A

Tier 8 = S = Master 

Tier 9 = SS = Grand (Profession)

Tier 10 = SSS = Sage

??? = EX = ???

The main character tends to refer to rank levels using letter grades, while the norm in this world is to use Tiers. Additionally, it's common to show respect by addressing someone as Master, Grand (Profession). and Sage. 

Low Tier Demon = B

Mid Tier Demon = A

High Tier Demon = S

Named Demons = SS

Grand Demons = SSS