Please refer to Section R201.3 and R201.4 where terms are not defined in the residential code, refer to the other FBC volumes for definition. If they are not found "such terms shall have ordinarily accepted meanings such as the context implies." The FBC Mechanical code defines Mechanical Equipment/appliance room. So a mechanical room would be a room that houses the HVAC system for that structure.
Saying that - is a residential HVAC closet a room? No. Regarding one and two-family dwellings, a closet or room containing an air handler is not considered a mechanical room.
The purpose of 16.2.2(4) is to prevent locations that contain contaminants to be used for return air.
