Mo. Rev. Stat. § 84.590

Promotions within the police department shall, as far as practicable, be made from lower ranks and, other factors being equal, preference shall be given to seniority. Unless the chief shall recommend and the board shall approve waiver of the requirement, a police officer to be eligible for promotion shall have served at least two years in the rank from which he is to be promoted and must be certified by the police physician as physically able to discharge the duties of the rank to which he is to be promoted. Patrolmen shall be considered of one rank without regard to their classification. It shall be considered a promotion when an officer is notified in writing by the chief of police that he has been assigned to a permanent position in another rank for which a higher maximum rate of compensation is prescribed by law; but assignment to a temporary or acting rank with a higher rate of compensation shall not be considered a promotion within the meaning of this section.