40 ILCS 5/11-211
If the balance in the expense reserve, the prior service annuity reserve, the child's annuity reserve, the duty disability reserve or the ordinary disability reserve, or either one of these is not sufficient to provide for expenses, annuities, or benefits chargeable thereto, the deficiency shall be removed by a transfer from the following reserves in the order stated: City contribution reserve; prior service annuity reserve; employees' contribution reserve; annuity payment reserve. When an excess exists in the said reserves to which a transfer was made, the excess shall be transferred from any of such reserves to the reserves from which a transfer had been made until the full sum previously transferred is restored. Interest of 4% per annum upon such transfers and retransfers shall be credited to the investment and interest reserve.
40 ILCS 5/11-211