What is "Fair play" on the Mystery board?

From version 1.9.22, Fair play (in the Codex, Mystery tab) lists what a careful mystery reader would hold against your book: a culprit who was never among the suspects, a clue that only appears at or after the reveal, a culprit who leads the suspicion too early or is left alone by elimination before the reveal, a red herring that is never explained, a run of chapters that plant nothing, a missing midpoint twist, or a suspect without a motive or opportunity. A green tick means that rule is fine; an amber mark is worth fixing. Under it, "Keep ... as an open thread to explain" asks the AI to explain a red herring later in the book.