70 ILCS 1205/9.2-4
Whenever revenue bonds are issued under Section 9.2-1 of this Article, all revenue derived from the operation of the tennis, handball, racquetball, or squash courts or zoos shall be deposited in a separate fund designated as the "Tennis Court Fund", "Handball Court Fund", "Racquetball Court Fund", "Squash Court Fund", or "Zoo Fund" of the park district. This fund shall be used only in paying the cost of operation and maintenance of the tennis, handball, racquetball, or squash courts or zoo, in providing an adequate depreciation fund, and in paying the principal of and interest upon the revenue bonds of the district issued under Section 9.2-1 of this Article.
A depreciation fund is a fund for such replacements as may be necessary from time to time for the continued effective and efficient operation of the tennis, handball, racquetball, or squash courts or zoo. Such fund shall not be allowed to accumulate beyond a reasonable amount necessary for the purpose and shall not be used for extensions to the tennis, handball, racquetball, or squash courts or zoo.
70 ILCS 1205/9.2-4