The basic idea is that Riviera has 341 rooms, but right now has about 100 rooms worth of timeshare stays sold. So on any given night 240ish rooms will not have a
DVC guest in them.
So DVC says basically “I declare 150 units are part of the time share” and then they can rent the other 190 rooms out for cash without having to wait for breakage or do advanced predictive math to do so. It also impacts those rooms participation in the costs and pay-in of the timeshare in ways I don’t understand well enough to explain.
Once Disney gets close to selling out the 147 units declared so far, they’ll declare another 50 or so into inventory.