What does "Three Graves and Five Codes" mean?

In the memory of the Chinese nation, China had four very famous works before the summer, which were called Three Graves, Five Classics, Eight Classics and jiusan.

It is said that the Preface to Shangshu written by Confucius is called "The Book of Fuxi (xi), Shennong and Huangdi", which is also called "Three Graves" and is also a road. The books of Zhuan Xu, Tang (Yao) and Yu (Shun) are collectively called the Five Classics. As for the books of Xia, Shang and Zhou Dynasties, although neither fish nor fowl, they are elegant and noble, but they are palm-sized, treasures of all ages and regarded as great teachings. Gossip is called eight kinds, and its meaning is also sought The ambition of Kyushu is called Jiuqiu. Mountains converge. It is said that all Kyushu, land and atmosphere converge in this book.

The question is, why are the books in the Three Emperors' era called "graves", the books in the Five Emperors' era called "Dian", the books in Fuxi era called "Suo" and the books in Yu era called "Qiu"? The answer in Preface to Shangshu is that "grave" has great righteousness, "canon" has eternal righteousness, "seeking" has seeking righteousness, and "hill" has gathering righteousness. Obviously, this is explained from the meaning of the above words, and it does not involve what materials are used as carriers in these ancient books.