40 ILCS 5/6-203
All gains from investments and all interest earnings shall be credited to the investment and interest reserve. All losses from investments shall be charged to this reserve. From this reserve shall be transferred all amounts due in interest upon balances existing in the city contribution, salary deduction, prior service annuity, ordinary disability, and the gift reserves.
Such amounts as may be necessary, according to the American Experience Table of Mortality and interest at 4% per annum, or the Combined Annuity Mortality Table with 4% per annum as to the assets or liabilities to which either table may be applicable in accordance with this Article for the purpose of establishing a balance in the annuity payment reserve equal to the liabilities chargeable thereto (including among such liabilities and in addition to all other liabilities of such reserve the present values of all annuities entered upon, or fixed and not entered upon, to be charged to such reserve) shall be transferred to the annuity payment reserve at least once each year.
That portion of the annual investment earnings on the fund's invested assets as required by this Section shall be transferred from the investment and interest reserve to the Supplementary Payment Reserve.
Any balance in the investment and interest reserve shall be either charged or credited to the Prior Service Annuity Reserve depending on whether a deficiency or surplus exists in the investment and interest reserve.
40 ILCS 5/6-203