The "Room Upgrade" paragraph I posted earlier in this thread included the line, "See terms and conditions for details." So I looked for the terms and conditions.
The "Rewards Terms & Conditions" page at
https://www.marriott.com/rewards/terms/elite.mi has this:
5. Complimentary Room Upgrade: Based on room availability at check-in and limited to a Member's personal guestroom. Upgrades may include rooms with desirable views, rooms on high floors, corner rooms, rooms with special amenities, rooms on Executive Floors, or suites. All upgrades are granted on a space-available basis, as determined at the time of check-in. Upgrades are subject to availability and identified by each hotel. Not available at Marriott Vacation Club.
Aside from adding the Marriott Vacation Club exclusion, it's not much different.