What is "Variety across stories" on the Quality review page?

From version 1.9.20, for a book that is a collection of stories, the Quality review page shows a card "Variety across stories". It checks the written stories against each other for what makes a collection feel repetitive: a character name used in more than one story, two or more stories that open with the same words, and a story that points at another one ("in the previous story"). Each finding names the stories concerned; click a story to open it in the editor. Characters you put in the book's Codex are treated as recurring on purpose and are never flagged. When nothing is found, the card says so. It costs nothing to run and needs no AI.