Spirit energy: Spirit energy is the main source of power in the world. Without the spirit energy, one cannot wield supernatural abilities. How hard one may train, for example, if a swordman endlessly trains, but in the end, if he doesn't have the spirit energy, it'll be a normal sword without any energy input. The same goes for the mages; however much mastery they may have over the aura, if they don't have spirit energy, it's all worthless.
Physical power - Swordsmanship
Magic - manipulation
Arts - focus and excellence in chants
According to that, spirit energy gets high and differs from others like normal beings and powerful beings via ranks. A S rank would have higher spirit energy, denser and stronger energy output compared to an A-ranked. There are subclasses for energy output for each type of class. Like a brute [melee, fist user] would use Eternal-flame path, while [Archers, tank, mages, summoner] would use Abyss path. If an eternal-path user tries to use an abyss path, the results would appear terrible. They will only gain power over a non-spirit-energy user. Its same for the abyss-path too.
Spirit-arts: The spirit arts, one of the most powerful and rare combat classes, are mastered by a few. 70% percent give up halfway through for its complexity and the strain it puts on one's body. Only 30% succeed and ascend to higher ranks. This class has the potential to ascend beyond SSS-rank. They're techniques with flow in chaotic spirit energy. For example, if a mage uses a flame spell its just a fiery one, but for the spirit art wielder, they'll be spiritual coiling flames, spiritual essence, they can invoke a spell to have its own will. Compared to a mage, a spirit artist would burn more spirit energy with their techniques. They can summon ancient beings to aid in battle, create barriers with effects, increase a weapon's potential with rune carvings, etc.