18. For the purposes of this Division, the following classes of units are established:(1) transport refrigeration units;
(2) refrigeration units having a power rating of less than 4 kW designed for commercial, industrial or institutional use, except refrigerated vending machines;
(3) air conditioning units having a power rating of less than 4 kW designed for commercial, industrial or institutional use, except refrigerated vending machines;
(4) refrigeration units having a power rating equal to or greater than 4 kW but less than 20 kW designed for commercial, industrial or institutional use;
(5) air conditioning units having a power rating equal to or greater than 4 kW but less than 20 kW designed for commercial, industrial or institutional use;
(6) refrigeration units having a power rating equal to or greater than 20 kW;
(7) air conditioning units having a power rating equal to or greater than 20 kW;
(8) refrigerated vending machines; and
(9) chillers.