Co-Writer
Co-Writer is the mode for writing an original novel — Book 1. The pipeline runs Book Bible setup, foreshadowing seeds, voice ledger creation, chapter pitch, draft, critique, consistency, polish, and Final Edit. It is the full journey from premise to finished EPUB.
Sequel Writer
Sequel Writer is the mode for writing the next book in a series. The bible from book one carries forward automatically — characters, world rules, timeline, prior events. The continuity check (a sequel-specific variant of the consistency pass) refuses to let book two contradict book one.
Authors who finish book one in BookWriter do not have to rebuild anything for book two. The series bible is already there.
When each fits
Use Co-Writer for any new world, new characters, new premise. Use Sequel Writer when you already finished book one in BookWriter and you want book two to inherit everything that already happened.