You can only define a public area by the criterion of exclusion. Any place that cannot be defined as a guest room is classified as a public area. The only other criterion is the traffic in these areas.
Public area is an umbrella term that encompasses the lobby, various restaurants and bars, cloak rooms, banquet halls, staircases, and elevators. Considering the size and number of these areas, it is abundantly clear that their maintenance is not an easy task.