"The reason why I picked this technique is because it is a Soul skill" Just as Amon said, Yang Chen noticed that the Soul's Timeless Archive was not a battle skill, but rather a Soul skill.
"The reason why it costs less than the other skills on the list is precisely because it is a Soul skill. Compared to the number of Battle Energy practitioners, the number of Soul energy Cultivator are much less"...
"It is mainly because anyone, even a person with the least amount of talent can train in battle energy. However, to train in Soul energy, one needs to be born with a talent for it and only then can you sense it. Soul energy is formless, colourless in the early stages and unlike the battle energy, one needs a special cultivation technique to even begin sensing it"