49 CFR 171.9: Rules of construction.

(a) In this subchapter, unless the context requires otherwise:

(1) Words imparting the singular include the plural;

(2) Words imparting the plural include the singular; and

(3) Words imparting the masculine gender include the feminine;

(b) In this subchapter, the word: (1) “Shall” is used in an imperative sense;

(2) “Must” is used in an imperative sense;

(3) “Should” is used in a recommendatory sense;

(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

(5) “Includes” is used as a word of inclusion not limitation.