(a) Restaurants, Hotels, Boarding Houses, Soda Fountains, and Similar Establishments. (1) Definitions. (A) Boarding House. The term "boarding house" as used in this regulation means any establishment regularly serving meals, on the average to five or more paying guests. The term includes a "guest home," "residential care home," "halfway house," and any other establishment providing room and board or board only, which is not an institution as defined in Regulation 1503 and section 6363.6 of the Revenue