What determined whether a Marriott hotel can call itself and qualify as a convention hotel?
Good question.
Marriott's
Guaranteed Late Checkout FAQ uses the term without defining it. For example:
6. What brands are participating in guaranteed late checkout for Gold and Platinum Elite members?
This benefit is guaranteed globally at all participating Marriott Rewards and Ritz-Carlton hotels, except at resort and convention hotels, where it is based upon availability. Marriott Vacation ClubŪ is excluded.
I suppose a "convention hotel" could be any hotel with its own convention facilities, as well any hotel near a convention center, or even a hotel with many guests attending a convention at a nearby hotel. Most full-service properties include meeting rooms and a "ballroom" (convention hall). Even Courtyard properties often have small "convention" facilities.
I hope that properties that value the patronage of Gold and Platinum guests will honor the late checkout benefit, except in rare cases when there's genuinely an unusual room turnover due to a convention.
Unfortunately, the lack of a clear definition of "convention hotel" seems to leave the door open for hotels to ignore this benefit if they choose to.