Story bible
The reference document holding a work's characters, places, rules and continuity — what television production calls the show bible.
A story bible starts as a convenience and becomes a necessity somewhere around the fiftieth thousand word. It is where you look up what colour a character's eyes were, whether the northern kingdom uses the same currency, and what the cost of magic was established to be.
The failure mode is not that writers do not keep one. It is that the bible and the manuscript drift: the draft moves on, the bible does not, and by the time you notice, you cannot tell which one is right.
Story bible in NovelKnow
The codex is the bible, but it is wired into the draft rather than sitting beside it. Entries carry aliases, mentions are re-scanned on every save, and state notes can anchor a fact to a specific scene — so "the capital" can become "the ruins" at chapter 40 without contradicting chapter 12.