What does "Does the book keep its promise?" do?

On the book's page (version 1.9.19 and later), open the "Promise" card and press "Does the book keep its promise?". One AI call reads the chapter titles and their plans against the title and subtitle and answers in a few lines: what the pair promises the reader, the chapters that deliver it, the chapters that drift from it and why, and a verdict with the one thing to change. When the chapters as a whole promise something different, or more, than the subtitle says, it also proposes a subtitle for the book as it is, with "Use it". Nothing is rewritten: you decide what to change, in the chapters or in the subtitle. The answer is kept on the card until you check again.