So we have a UM claim coming through & the attorney for the carrier requested all the sign down forms (fortunately we had them from 3 years ago.) I got to talking & here's the scoop. 1.) Insured has 1 car & ends up living with a relative & has a claim. If they're a resident relative they can technically collect on the relative's auto insurance for UM/UIM in addition to their own. This is where the stacking (of essentially two separate policies & limits would apply.) 2.) Insured inherits a car within the grace period of notifying the carrier. Ends up having a claim. Without rejection of stacking you'd be screwed.