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.