The hold back of adult dining slots for onboard dining is not always a guarantee. On at least one occasion we could not get any Palo dining slot in advance, we had the earliest boarding group and upon boarding, went directly to Palo to inquire about a reservation, and were told they were fully booked for all three nights. IMO, I think they hold back some slots for onboard booking most of the time, but not ALL of the time, particularly with concierge guests getting any request they want in advance.