Mo. Rev. Stat. § 36.320: Promotional register.

1. The director may establish and maintain such promotional registers and registers of eligibles for the various locations or divisions of service subject to this chapter pursuant to subsection 1 of section 36.030 as the director deems necessary or desirable to meet the needs of the service.

2. The time during which a promotional register or register of eligibles remains in force shall be determined by the director so as to best meet the needs of the service. The director may consolidate or cancel promotional registers and registers of eligibles as the needs of the service require.

3. In circumstances where there is a continuous need for substantial numbers of eligibles for a certain class of positions, the director may, after first establishing such a register, replenish the register from time to time by inserting the names of additional eligibles who are found to be qualified on the basis of determinations similar to those used as a basis for establishing the original register. The method for establishing, replenishing, and cancelling such a register may be determined by the regulations.