(1) Subject to the requirements of each retirement system, a vested member may purchase additional service credit. Members may purchase one full year of additional service for each five full years of membership service in the retirement system. A member eligible to purchase additional service may purchase months of service totaling 11 months or less.

(2) The board will include a member's additional service when calculating the amount of a benefit, but not for determining early or service retirement eligibility, except in the following cases:

(a) additional service purchased into PERS is required to be included when determining a member's early retirement reduction factor; and

(b) additional service purchased into SRS is required to be used when determining a member's retirement eligibility.

(3) A retired member who returns to active membership in the system from which they retired may purchase one-for-five service after at least 12 months of active service. The amount of one-for-five service which may be purchased will be based on the member's total membership service in the member's current retirement system.