Starting with the opening of the Grand Floridian - Disney offered members a more traditional timeshare ownership plan. Members could buy a fixed week (or Guaranteed Week) at a resort instead of just buying points. For example, if you wanted to stay at the Grand Floridian in a studio every year at the beginning of December, you could choose to buy week 49 of the calendar year, and you would have that week booked guaranteed. In order to do so, you have to pay a small (10%) premium in the number of points you have to buy to ensure that any future point adjustments would be covered for your week, but other than that - the week would be yours. You have to watch out though - for instance Thanksgiving week because it is the fourth Thursday in November does not always fall in the same week, so if you buy a week to go for Thanksgiving you may not always be there for Thanksgiving.
Fixed Week options to buyers of DVC direct deeds are at Aulani Resort and Spa, the Villas at Disney's Grand Floridian Resort & Spa, Disney's Polynesian Villas & Bungalows, Copper Creek Villas & Cabins, and now at Disney’s Riviera Resort.
Of course as these are pretty much run the same week every year. In fact, Disney started offering specific race-week contracts that also contained guaranteed race bookings with them (which of course you still have to pay for).
Well, the beauty of the fixed week option is that you can inform Disney that you are not going to use your fixed week, and they will convert your fixed week into standard points -including the extra 10%. You can do this any year you so desire.
Yes, there's no restrictions against buying/selling a contract, however the points/contract would then lose the member benefits that they had with Disney.
It's not common at all. There are probably only about 30-50 fixed week contracts sold at each resort since the Grand Floridian opened. Most of them are sold for late in the year. It's not widely advertised either as part of Disney's sales pitch is the flexibility. They also limit the number of fixed weeks that they will sell for a given number of available rooms because they do not want to block out members with standard points.
You can check which fixed week number for which dates, from the calendar here. Make sure to select Sunday as the first day, as DVC fixed weeks run from Sunday to Sunday.