35 ILCS 5/403: Effect of Determination for Federal Purposes.

(a) Reporting. To the extent not inconsistent with the provisions of this Act or forms or regulations prescribed by the Department, each person making a return under this Act shall take into account the items of income, deduction and exclusion on such return in the same manner and amounts as reflected in such person's federal income tax return for the same taxable year.

(b) Adjustment. A final determination pursuant to the Internal Revenue Code adjusting any item or items of income, deduction or exclusion for any taxable year shall be correct for purposes of this Act to the extent such item or items enter into the determination of base income.

(c) Identification of differences. To the extent required by forms or regulations prescribed by the Department, any person making a return under this Act may be required to indicate the item or items of income, deduction and exclusion which would enter into the determination of base income if this Act were amended to incorporate the Internal Revenue Code as amended and in effect for such taxable year.