Here is my understanding of how it really works early on:
1. For sale purposes, the resort is divided into "units." A unit can be a room or multiple rooms. Each cabin at CCV, since it is separated from any other cabins and rooms, will likely be its own unit, while the main building will likely have mostly units with more than one room.
2. Before sales begin DVD declares a number of units into the DVC system, and it is in one of those initially declared units in which an early purchaser will be assigned a real estate interest. Each unit has its own designated use year, so someone who adds on and gets a June use year will get an ownership interest in a different unit from a new purchaser who gets a different use year.
3. When you make a reservation, you can potentially be assigned to any room in the resort, including those in any units that DVD (the DVC developer) still owns and has not yet dedicated to DVC -- that arrangement is covered and allowed by a side agreement that exists between DVD and the related entities that control DVC reservations (the Disney Vacation Management Company and the Buena Vista Trading Company) The way reservations are actually divided between Disney and DVC members is on a percentage of dedicated units vs. non-dedicated. The units already dedicated to DVC constitute some percentage ownership interest of the total ownership interest at the resort, such as, for example, 20%. Thus, for any given day of the year, DVC members can reserve up to 20% of the room time available, while DVD, through the Disney central reservations system, gets to rent the other 80%.
4. For the most part, reservations at 7 months out by members during the early periods after sales begin are easier to get than when the resort approaches sell-out. That is because you have units dedicated to DVC that are only partly sold and thus purchasers of the new resort are not going to use up all the reservation time that DVC members can get. However, even that has many exceptions because new purchasers may skew their reservations to highly desired rooms (like ones with low point costs) or to highly desired times of year. For example, CCV, which, only days ago, opened for reservations by purchasers of CCV, already has studios booked full for early December, the highest DVC demand time of the year, and NYE, the highest DVC demand night of the year. It would not surprise me to see Thansgiving and some other times like maraton weekend in Jan disappear before DVC onwers of other resorts get a chance to book CCV. Once filled, rooms could open up later due to either cancellations or the addition of new units to DVC as sales progress.
Poly, as a prior example, after it first went on sale, actually saw a number of dates fill before non-owners could reserve in its first Oct through Decemeber period, but then in its second year, Oct to December was fairly open at 7 months out because many more units had been declared. Poly is actually now going the other way again, with more and more time disappearing before 7 month during that last quarter of the year (DVC's high demand quarter of the year). VGF, which has a limited number of studios that now are filled before 7 months out for much of the year, had only minor problems at 7 months out for a substantial period of time after non-owners could reserve there.