We own several resale fixed week WSJ VGV weeks. They are reserved for us automatically in the VSN system 18 months prior to check in. We do have to confirm the reservation through the Vistana (or formerly Vistana

) website or by phone prior to 12 months before check in to keep the reservations.
Here's the definition of an Ultra Premium Vacation Ownership Interest from the VSN Network Rules for Mandatory Results, Network Guide R-19, from 2022:
For comparison, here's the definition of a Fixed Vacation Ownership Interest from the same document:
The two definitions only seem to differ in the need to confirm the reservation, while the latter definition seems to require a positive act on the owners part to make the reservation my experience has been that the reservation is made automatically and I just need to confirm it. Since you indicated that you do not need to confirm the reservations by stating that you don't have to do anything if you want to go that week, that must be what the difference is. If you actually have to confirm the reservation the same as I do, then the term "ultra premium" is really just supposed to make you feel a lot better about what you own as compared to a lowly fixed week.
As for the platinum plus part, that's the season name the seller is using and may or may not match what they are really offering. If what you are looking at is in Virgin Grand, it will be a fixed week/fixed unit ownership as that is how they were all sold.
If it is in Coral Vista, the seasons for what they are selling are Diamond, which lines up with platinum plus, and Resort, which combines platinum and gold plus seasons. It really only matters for when you have the home reservation period, though; the number of options needed for a reservation would follow the number listed in the StarOptions chart. (N.B., in the gold plus season, which is the latter half of the Resort season or weeks 34 to 50, the options for a 3 BR LO are 125,000 while a 2 BR is 95,700 and a studio is 44,000 and the latter two don't add up to a 3 BR LO. When you actually go to make a reservation there you will note that there are some 2 BR weeks listed that don't cost 95,700 but instead a lower amount that I can't recall. Similarly, some studio weeks cost less than 44,000. The lower costed ones are the component parts of the 3 BR LO; a 2 BR that costs 95,700 in the gold plus would either be a standalone 2 BR or a 2 BR Loft.) I'm not sure how the Bay Vista and Sunset Bay phases are set up.