Illinois taxes income earned for personal services performed while in Illinois (even if you're a nonresident). This includes remote work for an employer or clients based outside Illinois if you were physically in the state while doing the work.
Register as a new employer using the New Business Registration application (Form REG-1) on MyTax Illinois. This will create a withholding account with the Department of Revenue. Alternatively you can file forms REG-1 and UI-1 together by mail.
Box 17: State income tax withheld. Box 17 reports the total amount of state income taxes withheld from your paychecks for the wages reported in Box 16. Box 18: Local wages. Box 18 reports the total amount of wages subject to local income taxes.
California (CA) State Withholding Tax Laws Non-California residents, including U.S. citizens who are residents of other states, are subject to State income tax withholding of 7% of gross if the total payments excel $1,500 during the calendar year.
IL-1040 Individual Income Tax Return.
You must submit Form IL-W-4 when Illinois Income Tax is required to be withheld from compensation that you receive as an employee. You may file a new Form IL-W-4 any time your withholding allowances increase.
Box 17 (State Income Tax): This amount represents the total of state income taxes withheld from your paycheck for the wages reported in Box 16.