Class

Heaven, Earth, Xuan, Huang (天、地、玄、黄) classes/tiers are used to classify various things throughout the novel including
* Soul root
* Spirit Blood
* Spiritual Strength
* Battle Skills
== Details ==
Ordinary Battle Skills and cultivation techniques were split into Heaven, Earth, and Mortal classes. But these three classes referred only to Xiantian experts and below.
Xiantian experts and above could use their energy to control the energy of heaven and earth, transforming it into all kinds of runes that could release mountain-toppling power. Such attacks had broken through the limit of Battle Skills and were called magical arts.
Magical arts were not split into high or low grades. There was no system to measure that. Their power depended mainly on whether the user could control it properly.
At the Xiantian realm, people fought by using their energy to control heaven and earth's energy. That allowed them to release an even more terrifying power. Magical arts were what allowed them to control that energy. If you wanted to control more energy, it would depend on your abilities.
==References==