In the Index page a heading says "subject of the book" and it is not in my index. Why?

A printed index does not list the subject of the whole book: in a book about Byzantium, "Byzantium" would point to almost every page and help nobody. When a heading is mentioned in a large share of the book, BookyAI keeps it in your list but switches it off and labels it "subject of the book". You find these under the "Kept out" filter on the Index page. If you want it in the index anyway, click the eye icon on its row to switch it back on.