I'm sure you're correct in that they represent different phases of the resort. However, I can't help decipher what code is what section, or whether they actually put you in the section represented by the code.
For instance, at Marriott Williamsburg, they have MMC (Manor Club) and MSE (Sequel). Both my stays there were for MMC, and I was put in that section each time.
But Marriott Grande Vista has MGV and MGR, which I assume represent the older and newer sections of the resort. In several stays, I had both MGV and MGR reservations, but have always stayed in the older section.
I assume you are familiar with Vistana Resort, but in case not, it was developed over a period of maybe 25 years, with maybe 6 or 7 different phases. Some phases are much newer and nicer than some of the others. The newest phase is probably less than 5 years old, and was under construction about the time Sheraton become involved. The two phases prior to that (sorry, can't recall the names), are probably the most desirable phases.