There is no explanation in the book, but the word Yin itself can be understood in this way: the original meaning of the word "釞" can be literally translated as "yellow earth"; It is a "yellow land where people gather together". I feel that the author thought he had a kingly demeanor, so he gave him the name of a king of the earth. As a prince, he is the successor of the King of the Earth, so he is called Cheng Yin.