Class Ranking of Mutants (Not entirely precise since it wasn't based on fixed indicators. One can only know after the mutated nucleus of the monster was retrieved, power judged by the quality of super energy contained within) :

Highest to Lowest: SSS — SS — S — AAA — AA — A — B — C — D

(Whatever's under D is trash. Better not absorb it lest you evolve into one)

Power Ranking of Ability Users (Purely based on energy power, examined by a Power Assessment Gadget. With this, lower-ranked users might win against higher-ranked ones, but the gap difference will become larger and larger in as the level grows, which make it almost impossible to happen in the later stages) :

Lowest to Highest: 1st Rank ~ 13th Rank

Comparison of strength – Mutants : Ability Users (Usually attack in groups)

C-Class = Under 3rd Rank

B-Class = 3~4th Rank ability users (Usually attack in groups)

A-Class = 5~6th Rank ability users

AA-Class = 7~8th Rank ability users

AAA-Class = 8~9th Rank Ability Users

S-Class = 9~10th Rank Ability Users

SS-Class = 11~12th Rank Ability Users

SSS-Class = 13th Rank Ability Users

Weapon Mastery Ranking (Usually, only weapon wielders and non-ability users care for this ranking) :

Highest to Lowest: Expert — Master — Proficient — Advanced — Novice

Orders of Safety Region (Based on listed experts contained within the place as well as how likely it is to suffer from mutant attacks) :

Highest to Lowest level of threat: Red — Orange — Yellow — Green

Other Terms:

Super Core/ Mutated Nucleus : Meant the same and works similarly, what makes it different was Super Core belongs to Ability Users while Mutated Nucleus belongs to Mutated Monsters.

(Ability users hunt mutated monsters for their cores. Although it'll be a slow and arduous process, they could refine their own core with the energy derived from mutated nucleus. However, unlike ability users, mutants wasn't interested in human's super core. They would usually hunt their own to absorb their prey's energy, done by consuming them whole)