The Nine Yin True Scripture is not a cultivation method designed for the human race after all, so the processes in each stage of Body Refinement and shedding of the skin are not quite the same. However, according to the Daoist Scriptures of the Supreme True Palace, when human Qi Refining Warriors reach a certain stage, all of them must undergo Core Formation.
In the Supreme Dao, it's called the Golden Core, while in the Divine Sect, it's called Divine Core, but the difference isn't significant. Essentially, it's the nucleus formed by the condensation of Daoist Breath, blood energy, and energy qi clusters.
It's like taking the Dao Qi, which was not well digested and scattered outside the body during the Refining Essence into Qi process, and using it to refine and cleanse the Golden Core or Divine Core. When in urgent need, one can then draw upon the cultivation level of the Golden Core, so they aren't left powerless.
