Illinois Administrative Code
Title 86 - REVENUE
Part 100 - INCOME TAX
Subpart M - BUSINESS INCOME OF PERSONS OTHER THAN RESIDENTS
Section 100.3340 - Business Income of Persons Other Than Residents (IITA Section 304)

In filing returns with this state, if any person departs from or modifies the manner in which income has been classified as business income or nonbusiness income in returns for prior years, such person shall disclose in the return for the current year the nature and extent of the modification. If the returns or reports filed by a person for all states to which such person reports under Article IV of the Multistate Tax Compact or the Uniform Division of Income for Tax Purposes Act are not uniform in the classification of income as business or nonbusiness income, the person shall disclose in its return to this state the nature and extent of the variance.