What is "Confirm title and subtitle for publishing"?

From version 1.9.19 the "Promise" card on the book's page has a button to confirm the title and subtitle as they are. It is the line "Title and subtitle" of the "Ready to publish" list: still working until you confirm, ticked afterwards. Exports and the cover use the title and subtitle either way; the confirmation is the reminder to settle them before the book goes out. Any later change to the title or the subtitle reopens it, and "Reopen" on the card does the same by hand.