Yes it is IL income and MI income. You should prepare the nonresident state first. Then prepare your resident state with a credit for income double taxed. You get credit for the lower state tax on the lowest taxable amount.
Each state calculates taxable income differently.
Each state has its own tax rate/ system.
You get the lowest of both categories as a tax credit
IL instructions page 3 say that you should file an IL return if you have taxable income as a nonresident.
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