The first contract you purchase will be a "master" contract with a contract number and a ".000" extension, for example: 123456789.000. If you add on more points with the same Use Year (either through Disney or via resale and at the same resort or a different resort) it will be a separate contract but tied to your master contract. It will have the same base number but a different extension: 123456789.001. Each contract has identifying information including the home resort for that contract and whether it was a direct or resale purchase. If it is a resale and was purchased after 3/20/11, Member Services will see that those points are restricted and cannot be used for cruises for example.
When booking at a
DVC resort, if your master and add-on are for the same resort they function like one big contract. If you are booking in the "home resort" window (11 months to 7 months plus one day from check in) you can use points from either or both contracts to make your reservation. If your add-on is for a different resort than your master, you cannot combine the points when booking in the home resort window because they are for different home resorts. You can use points from resort A to book at A 11 months out and use points from resort B to book at B 11 months out but you cannot use points from A to book at B until 7 months from check in, even if you are an owner at resort B.
The home resort booking window should be called the "home resort points" booking window since it doesn't matter where you own, what matters is having enough points for resort X in your account if you want to book there 11 months out.