It actually has nothing to do with DDP, but rather on-site guests.
Everyone (on-site, off-site, day-trippers, etc) can book ADRs at 180 days from the date of dining. The 180+10 comes into play for on-site guests only and allows them to book up to 10 days worth of dining at the 180 day mark before they check in.
So, instead of having to call/log on every day for your vacation's ADRs, you simply have to call/log on once, 180days before your check-in date.
If you are staying more than 10 days, I believe you can make day 11 on the day after your 180 day mark, and so on (don't quote me on that one, I haven't been lucky enough to be able to take a longer than 10 day trip ).
The only difference with the dining plan is if you make a reservation for CRT, or something that requires payment at the time of the reservation, it will not charge your card. Being a DVC member doesn't change anything as far as reservations are concerned.
I think that after the 10 days, the reservation system just goes back to 180 days out from the actual day of the reservation.
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