49 CFR § 192.15 - Rules of regulatory construction.
(a) As used in this part:
Includes means including but not limited to.
May means “is permitted to” or “is authorized to”.
May not means “is not permitted to” or “is not authorized to”.
Shall is used in the mandatory and imperative sense.
(b) In this part:
(1) Words importing the singular include the plural;
(2) Words importing the plural include the singular; and
(3) Words importing the masculine gender include the feminine.
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