I found my answer on MGM's website. Always best to go to the source.

Express Comps are the fastest and most convenient way to self-comp at your favorite M life Rewards Destination. Members may earn up to $5,000 in Express Comps and NOIR Members may earn up to $10,000. Express Comps may be used for your hotel stay, at food and beverage outlets, entertainment venues, or to purchase your M life Rewards Moments. Members do not earn Tier Credits when using their Express Comps.