After realizing the class of Primals, Wang Wei realized he may be in trouble. During their training, Wu Hong divided Primal into six tiers to facilitate a better understanding of his standing and the amount he needed to learn. It's also served as a way to prevent him from getting cocky.
The lowest class is the E-Class, and this level is reserved for people like Water Immortals, who are flawed Primals. This level did not exist until a few thousand Yuan Epoch ago, when they used the latter as a pawn to control the Limitless Exchange Hall. E-Class Primals' strength is not that much different from the above class, but their flaws mean a significant weakness that can be easily exploited.
