35 ILCS 620/7
Every taxpayer under this Act shall keep books, records, papers and other documents which are adequate to reflect the information which such taxpayers are required by Section 2a.2 of this Act to report to the Department by filing annual returns with the Department. The Department may adopt rules that establish requirements, including record forms and formats, for records required to be kept and maintained by taxpayers. For purposes of this Section, "records" means all data maintained by the taxpayer, including data on paper, microfilm, microfiche or any type of machine-sensible data compilation. All books and records and other papers and documents required by this Act to be kept shall be kept in the English language and shall, at all times during business hours of the day, be subject to inspection by the Department or its duly authorized agents and employees. Books and records reflecting kilowatt-hours of electricity distributed during any period with respect to which the Department is authorized to establish liability as provided in Section 5 of this Act shall be preserved until the expiration of such period unless the Department, in writing, authorizes their destruction or disposal at an earlier date.
The Department may, upon written authorization of the Director, destroy any returns or any records, papers or memoranda pertaining to such returns upon the expiration of any period covered by such returns with respect to which the Department is authorized to establish liability.
35 ILCS 620/7