Tom has it right; with the SFX privilege weeks you can exchange back into Vidanta up to your ownership level for just a bit more than a typical usage fee. This includes during elite weeks (Xmas, Easter, etc.). I got one for this coming Xmas and think it was $75 more than the usage fee.
You can also use them to go to other resorts.
If you aren’t looking for an elite week reservation and have other TS, SFX typically gives you bonus weeks when you deposit those with them; with those bonus weeks you can get into up to a 2 BR GL suite for about half the cost of a usage fee plus a $75 resort fee. SFX typically only has inventory up to that level; for higher levels they have to set up a conference call w/Vidanta customer service to make privilege week reservations. Dirty little secret: the folks at Vidanta customer service are paid by the reservation and will probably make one for you if there’s availability without having you go through SFX if you tell them that SFX doesn’t have the inventory. I haven’t actually done that, but the folks on the phone there have told me to just call them if SFX can’t get what I want. Not sure if it would be coded differently as a privilege week if you went that way; if not, you get the addenda privileges.
Someone posted a list of prices a while ago for the 2017 usage fees. Haven’t seen the 2019 ones yet.