Not usre about Marriott either, but I own in the Gold season....those are weeks 11-20 and 41-50 with Divi Resorts. My specific weeks are 44 and 46 (working on 45...), so I can go anytime from week 11 through week 50 and only pay a tranfer fee if not going on my specific weeks.However if I wanted to go during Platinum which is weeks 51, 51 and 1-10 I would have to pay a $300 upgrade fee. If you own in Silver (21-40) and want to switch to either you pay a fee.....Platinum is the only one that doesn't pay an upgrade fee, or if you buy a floating week you do not have to pay a fee...the prices are higher to buy depending on which season you buy in....So if I had my week 46 in 36 it may be cheaper, but I would have to pay more to move it around...
Then...if you want to go to Hawaii for instance you must belong to either Interval International or RCI...these are companies that you can trade your weeks through to go to other places if you prefer. There is a yearly fee to belong and a fee to exchange...I haven't traded for awhile, but it used to be about $100 to trade in US and $150 to trade internationally..which the Caribbean is considered....(If you belong to Divi you can trade to any of their Caribbean resorts for just a trade fee of $90)...I have used II(interval International) a few times and have always gotten what I requested and have nevr been disappointed about the hotel upon arrival.....
Hope this answered some of your questions....

Aruba, my home away from home !!