What is the working subtitle, and how do I set it?

From version 1.9.19 the New Book form has a "Working subtitle" field under the working title. It is the promise of the book: what the reader gets, knows or can do after reading it, in one line. It is optional and it can change at any time. "Suggest title and subtitle" next to the field proposes five title and subtitle pairs from your topic, genre and audience; one click fills both fields. On the outline review, before any chapter is written, the title and subtitle sit at the top to settle. On the book's page, click the line under the title to edit the subtitle at any time, and the "Promise" card in the right column shows the pair, how often it changed, and proposes pairs from the chapters actually written.