From my II history (I once owned a one-BR in Cascades -- non-SVO):
Sheraton Vistana Resort
VIT
Unit: 2107 (1 bedroom)
Week: 9
I am 99% sure that VIT was "Lakes and Cascades Only" at one point in time. Things could have changed over the years ...
Once, many years ago, Starwood really screwed up a deposit so gave me a 2-BR Fourth of July week in Fountains instead of my normal one-BR deposit. It was coded VIO in my II account -- and Fountains was NOT part of SVN at that time. Again, things could have changed ....
I'm not sure it matters -- I think they only use the codes for inventory control purposes. Somewhere on TUG, there's a list of the phases by check-in date. I think it's safe to request anything that matches the size/occupancy and check-in date and I'm sure they'll try to fulfill the request. I requested a renovated Lakes unit on my last visit and that's what I got.
I checked into a fixed week/fixed unit timeshare today. I have stayed at this place ~15 times and always received the unit on my confirmation. Today -- I got a different unit ... it has a few steps ... my guess is that the people originally assigned to it couldn't handle the steps. Either way, it's fine with me! I've never felt that I had the right to demand any particular unit when exchanging.