Illinois Statutes Chapter 35. Revenue § 5/1401. Promulgation of Rules and Regulations

(1) For taxable years ending before December 31, 1993, the Department shall make, promulgate and enforce such reasonable rules and regulations, and prescribe such forms as it may deem appropriate, to permit all of the taxpayers that are corporations (other than Subchapter S corporations) having the same taxable year and that are members of the same unitary business group to elect to be treated as one taxpayer for purposes of any original return, amended return which includes the same taxpayers of the unitary group which joined in the election to file the original return, extension, claim for refund, assessment, collection and payment and determination of the group's tax liability under this Act. For taxable years ending on or after December 31, 1987, corporate members (other than Subchapter S corporations) of the same unitary business group making an election to be treated as one taxpayer are not required to have the same taxable year. The rules, regulations and forms promulgated under this subsection (b) shall not permit the election to be made for some, but not all, of the purposes enumerated above.

(2) For taxable years ending on or after December 31, 1993, the Department shall make, promulgate and enforce such reasonable rules and regulations, and prescribe such forms as it may deem appropriate, to require all taxpayers that are corporations (other than Subchapter S corporations) and that are members of the same unitary business groups to be treated as one taxpayer for purposes of any original return, amended return, which includes the same taxpayers of the unitary group which joined in filing the original return, extension, claim for refund, assessment, collection and payment and determination of the group's tax liability under this Act.