UIM coverage applies when the at-fault driver has insurance but not enough to cover your damages. If the at-fault driver has $25,000 in liability coverage and your damages are $100,000, your own UIM coverage can pay the $75,000 gap up to your policy limit.
UIM coverage is usually included in auto policies but the limits depend on what the policyholder bought. Higher UIM limits cost only modestly more and are some of the highest-value coverage you can buy.
Stacking UIM across multiple vehicles is allowed in Missouri but generally not in Illinois (subject to specific policy language). Coverage analysis on UIM cases involves reading the policy carefully.