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.