you can book starting 180 days out from the first day of your trip.
+10 is that as long as you're staying at a Disney resort, whether cash or
dvc points, you can book up to 10 additional days beyond the 180 at the same time you do your first day.
In other words you can book the full vacations ADR on the first day rather than having to go back online every day 180 days out to make your trip reservations. A small advantage but will let you book high demand places before the general offsite public has access...