One's ranking is based on the amount of mana and control over the elemental affinities they have. The ranks:
F-Rank (the lowest rank)
E-Rank (normal animals)
D-Rank (average)
C-Rank (tank)
B-Rank (can destroy a city)
A-Rank (Super solider)
S-Rank (atomic bombs)
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Skills and Special Techniques:
Skills are a kind of technique. Magical essence is not always required to use skills. Skills are mostly dependent on one's knowledge. For example, a man doesn't have a leg, but he knows how to build a prosthetic leg. So, he makes his own prosthetic leg. Here, the prosthetic leg is a skill, and the man developed it because he knew how. Now, if he developed the skill on his own, it would be called Skill Creation, but if he learned it from someone else, it would be called Skill Learning.
When a few skills are combined to use them as a single skill, it is called a Special Technique. Like, after the man created a prosthetic leg, he combined his knowledge about robots and added a motherboard, some wires, and a program to turn the normal prosthetic leg into a robotic prosthetic leg. Now, he can kick, run freely, and break other people's legs. Here, the robotic prosthetic leg is the Special Technique, which the man created, combining his other skills.
If he had higher knowledge, he could have made a nuke. Which would be the Special Technique:
AtomicBoom: (Special technique, Rank: S. Atomic Boom is created by combining the skills, Radiation Disaster and Chain Explosion.
Usage: Can explode a specific area for a limited time repeatedly after one use.
Effects: Will turn everything lower than A-Rank into ashes)
Skills:
One last chance: (Skill, Rank: B, let's the user put all his strength and mana into a single attack or ability.
Effects: Drain all the magic essence user has and exhaust the user to his limit.)