Ban Gu, a historian and litterateur of the Eastern Han Dynasty (AD 32-92), classified ancient and modern figures into the "nine grades scale" in his "Hanshu Ancient and Modern People Table", which was divided into three grades: upper (upper wisdom), middle (middle people), and lower (lower fool). In each level, it can be divided into upper, upper middle, upper lower, middle upper, middle middle, middle lower, and lower upper, lower middle, and lower lower.
It is also said that people are divided into ten grades, which means that people are divided into high and low. In the past, it was said that:
One official; Two officials; Three monks; Four ways; Five medical sciences; Six jobs; Seven craftsmen; Eight prostitutes; Nine Confucian scholars; Ten beggars.
Generally speaking, "three, six, nine and so on" is to distinguish and belong to the categories and levels of the world and even the universe. In the narrow sense, it refers to the distinction between the levels of material, ideological, professional, etc. that people have.