What does "In a copy of this book" do when I regenerate all the chapters?

From version 1.9.21, "Regenerate all" on the book's page can write the new chapters into a copy instead of the book itself. BookyAI first makes a full copy: the manuscript, the outline, the Codex, the cover and the pictures. Then it writes every chapter of the copy again with the engine you chose. Your original book stays exactly as it is, and the copy appears in your Library with a small "Copy" label (hover it to see which book it was copied from), so you can read both side by side. Some things are deliberately left out of the copy because they belonged to the old text: the exported files, the audiobook, the Versions history, the quality review and the book index. The copy is also not part of the original's series. A good way to try another AI engine on a whole book without risking the one you have.