Uninsured/underinsured (UM/UIM) insurance coverage is usually sold as an optional clause to add to a standard auto insurance policy. If you have UM/UIM insurance, then it can provide additional coverage if the person who hit you does not have enough insurance coverage to pay for all of your damages, whether they have no insurance or just a limited policy. To use UM/UIM insurance, you must file against your auto insurance provider, which can complicate matters and pit you against the insurance companies that is supposed to be helping you.