20 ILCS 2515/3
Any written informal ruling, opinion or letter issued by the Department in response to an inquiry or request for any opinion from any person regarding the interpretation of, liability incurred under, coverage of or applicability of any revenue laws administered by the Department shall be maintained as a public record in the Department's principal office, and made available for public inspection and copying. If the informal ruling, opinion or letter contains trade secrets or other confidential information, where possible the Department shall delete such information prior to its being made available for public inspection. Whenever such informal ruling, opinion or letter contains any policy of general applicability, the Department shall formulate and adopt such policy as a rule in accordance with the provisions of The Illinois Administrative Procedure Act.
20 ILCS 2515/3