Just to be sure everyone is clear here.....the 180+10 is not 10 days earlier than you would normally be able to book, making it possible to call 190 days before your trip. You call on the 180 day mark (6 months before your checkin date) and then you are able to book up to the first 10 days of your trip with that one call.
It used to be that you called on the 180th day, for your arrival day, then you had to call every day after that for each succeeding day...it was a huge pain. Now, you don't have to call several times...you can do it all at once, on that 180th day, for your entire trip. Just didn't want anyone thinking they could call 190 days before their arrival date if they are staying on-site.