The State of Illinois allows a subtraction from income for certain military pay earned, which is claimed on Schedule M.
Per the Instructions for the Schedule M, page 3:
Enter the amount of military pay that you received from the U.S. Armed Forces or the National Guard of any state, or that you received as a dual status military technician.
You may not subtract the following:
For more information, see Publication 102 Illinois Filing Requirements for Military Personnel.
Attach Form W-2 showing your military pay.
To enter this in TaxAct: