What is the "Ready to publish" list on the book's page?

From version 1.9.19 the first card in the right column of a book's page is "Ready to publish". It is computed from the book itself: the chapters written, the cover, the front and back matter, the quality review, the title and subtitle, and at the end the export. A ticked line is done; a line with a verb on the right (Generate, Choose, Run, Confirm) is something you can do now, and clicking it opens the card where it is done and lights it for a moment. Lines that come later (for example the cover, before any chapter exists) are greyed out. The count at the top, "3 of 5", is what can be done now. When every line is ticked, the last one becomes "Ready. Export the book" and exports every format in one click. Exports work at any time either way: the list is a reminder, not a lock.