40 ILCS 5/20-101

There is established a plan for the continuity and preservation of pension credit, in accordance with the provisions hereof, in the case of employees transferring employment from one governmental unit to another. The purpose of this plan is to assure full and continuous pension credit for all service in public employment which is covered by a retirement system. The acceptance of the provisions of this Article, shall be optional with the employee, or in the event of his death, with his survivor; however, the provisions of Section 20-120 shall be applicable to every person who applies for benefits from 2 or more retirement systems covered by this Article.