In Article 4.1.5.5., “accessible” refers to the lack of a physical barrier that prevents or restricts access by vehicles or persons to the site in the context of the specific use.
Information on the design of roof parking decks and exterior areas that are accessible to vehicular traffic can be found in the Commentary entitled “Live Loads” in the “Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B)”.