Shared yards or decks
On so many house hunting shows I hear that yards, and more commonly, decks, are "shared". My question is about who decides how and when they are used, especially in smaller buildings. For example, if four units share a rooftop deck is their time split? by days? hours? by reservation? What is someone wants to sit out and quietly read, can another resident come in with ten guests and have a party? play music?
Or, is it shared by space, with each resident having a certain area, seen or not seen by others?