Illinois bars UM stacking by statute and lets your UIM policy limit it
Illinois's rules are in 215 ILCS 5/143a for UM and 143a-2 for UIM. The minimum limits are in 625 ILCS 5/7-203, and the injury deadline in 735 ILCS 5/13-202.
Read how stacked and single-limit coverage differ in Illinois, check what sections 143a and 143a-2 say, or see how UM and UIM pay differently.