You only get OBB benefits for ONE future cruise, so either a specific future date OR a placeholder.
You can book multiple future cruises while on board but would only get the OBB benefits for one of them.
(note: if you are sailing B2B as we did you can book a placeholder/future cruise, one for each leg... so went ended up with 2 placeholders each with a slightly different "use by" date; this morning I used them to book another B2B).
SW