What is a Codex?
Think of it like a scroll. You can unroll it, write on it and then after you are done, you can roll it back up and stuff it away in your pouch. In Underlight, a codex could be made using the Art of Inscribe, which was only obtainable at orbit 20. Through this art one could jot down one's thoughts and share them with others. Because it was only obtainable at Orbit 20, or 2nd sphere, many seeking teachership could not be ascended until they were at least able to inscribe their own tasks. This was a major mile stone to achieve for any dreamer.