Starwood sends their hotels an ordered "suggested" upgrade list. That list is ordered by a number of attributes like revenue generation, nights stayed, rate, etc. The details behind the list are proprietary and not published, so we don't know exactly what goes into it.

Each hotel can use the list or not as it chooses, but Starwood does try to give some guidance to each hotel. I wonder if Marriott has anything similar or do they let each individual hotel decide completely on its own?