Open space. A minimum of 15% of the gross tract area shall be devoted to usable open space. Usable open space may be provided in a flexible manner and may include, but is not limited to, courtyards, rooftop amenity areas, landscaped plazas, pedestrian greens, landscaped buffers, and stormwater management features designed as accessible or visually integrated amenities, as approved by the Planning Board. The Planning Board shall determine compliance based on overall site design, functionality, and contribution to resident livability, rather than the specific form or location of the open space.