49 CFR §171.9

1. (a)In this subchapter, unless the context requires otherwise:
   1. (1)Words imparting the singular include the plural;
   2. (2)Words imparting the plural include the singular; and
   3. (3)Words imparting the masculine gender include the feminine;
2. (b)In this subchapter, the word: (1) “Shall” is used in an imperative sense;
   1. (2)“Must” is used in an imperative sense;
   2. (3)“Should” is used in a recommendatory sense;
   3. (4)“May” is used in a permissive sense to state authority or permission to do the act described, and the words “no person may * * *” or “a person may not * * *” means that no person is required, authorized, or permitted to do the act described; and
   4. (5)“Includes” is used as a word of inclusion not limitation.