Book on board (BOB)

What is a book on board (BOB) and how does it work? → Royal Caribbean Next Cruise Programme → MSC Future Cruise and Open Booking → P&O Cruises Book Onboard Programme → Princess Cruises Future Cruise Deposit → Norwegian Cruise Line - Cruise Next Credits → Celebrity Cruise - Book Now or Book Later → Cunards Book Onboard → Fred. Olsen Future Cruise →