Never heard of a "guarantee" for a specific portion of the ship. It has only been recently that Princess started to offer the lower prices for the 'guarantee' cabins, that had previously only been offered by discount consolidators. Prior to that, you always had to select a cabin if booking online, as well as booking through most travel agents. Was also told by a future cruise consultant that the 'guarantee' cabins will almost always be forward or aft, and most likely on the higher decks, i.e. the lower priced ones that people would not pay to reserve. In our case, we have to reserve a cabin because we require an ADA cabin.