Occupancies within buildings shall be separated from one another as follows:
a. Spaces classified in occupancy groups having a higher fire index, as listed in table 3-1, than the fire index of the occupancy group classification of the building, shall be separated from ad joining spaces by construction meeting the fire-resistance rating requirements for fire divisions under the provisions of subdivision (a) of section 27-339 of article five of subchapter five of this chapter. Such occupancies shall, for the purposes of this code, be classified and treated as separate buildings (hereinafter referred to as "building section").
b. Spaces classified in occupancy groups having the same or lower fire index, as listed in table 3-1, than the fire index of the occupancy group classification of the building, shall be separated from adjoining spaces by construction meeting the fire-resistance rating requirements for fire separations under the provisions of subdivision (b) of section 27-339 of article five of subchapter five of this chapter.