Words used in this code shall have the meanings as defined below. Words in the present tense include the future tense. The singular number includes the plural and the plural number includes the singular. The word "shall" is always mandatory and the word "may" is permissive. The words "used" or "occupied" include the words "intended, designed, or arranged to be used or occupied." Certain terms or words not defined shall have the meaning customarily assigned to them as defined in Webster's Ninth New Collegiate Dictionary.