On property: You can book up to 10 days once your first day hits the 180 day mark. This is a nice advantage, because you can book reservations that are 185, 187 days, thus you have a better chance of snagging what you want.
Off property: You can only book when that given day hits the 180 day mark, so you need to book one day at time for your reservation.
For me, I was flexible in planning out my itininary, as there were some must have reservations and so I moved some things around. I'll also look to align what parks you go to with what reservations you have so you are not compelled to buy park hoppers.