Disclaimer:</u> Grade 1 to 3 Planets are Mortal Layer planets. Grade 3 to 6 are Immortal Layer Planets. Grade 7 to 9 are Divine Layer Planets. They correspond to each Rank (not level) in cultivation. If you want more information, you can check the chapter at the end of Arc 1 (between chapter 14 and 15), with a lot more descriptions about them.