49 CFR §390.7

1. (a)In part 325 of subchapter A and 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;
   3. (3)Words imparting the present tense include the future tense.
2. (b)In this subchapter the word—
   1. (1)Officer includes any person authorized by law to perform the duties of the office;
   2. (2)Shall is used in an imperative sense;
   3. (3)Must is used in an imperative sense;
   4. (4)Should is used in a recommendatory sense;
   5. (5)May is used in a permissive sense; and
   6. (6)Includes is used as a word of inclusion, not limitation.