In order to distinguish between District businesses and for the purpose of calculating and applying the amount of assessments owed, the following definitions shall apply:
A. Retail Businesses. "Retail Businesses" include all businesses not covered by other definitions set out in this section, at least fifty percent (50%) of whose gross income is derived from "retail sales" as that term is defined under the California Sales and Use Tax Law. The fact that a substantial part of its business consists of sales other than retail sales does not exclude said business from this classification so long as such