When you find a resort you are interested in buying you can ask here on TUG and someone will be able to tell you how the weeks work at that resort. Even within resorts sometimes you have some fixed weeks and some floating weeks.
If I am interested in buying ANY WEEK, ANYWHERE, I always call the resort mgt and give them the week/unit number and ask questions about the week I am considering buying. Is week xx unit number x fixed or floating, are the MF's are current, when is this week next avaialbel for use (sometimes future weeks have been deposited with exchange companies), if there are any past or current assessments due now on this unit, or if there are any upcoming assessments. I also try to ascertain what view this unit has if it is a fixed week (pool, ocean, parking lot),what units and time frames I can reserve if it is a floating week ( 1-52, just winter,just summer, garden view only, best available, etc), square footage, number of bed rooms/baths, type of beds (king, twins, etc), how much annual MF's are, and anything else that I need to know to make an informed purchase. I never place a bid on an ebay auction until I have contacted the resort and asked questions about the exact unit that is for sale. Once you own it, it is too late to find out that it isn't what you thought you had purchased.