Section 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.Amdt. 171-32, 41 FR 15996, Apr. 15, 1976, as amended by Amdt. 171-32A, 41 FR 40630, Sept. 20, 1976; Amdt. 171-121, 58 FR 51528, Oct. 1, 1993; 75 FR 60338 , Sept. 30, 2010