Underinsured Motorist Coverage or UIM is different from Uninsured Motorist Coverage. UIM coverage is on your personal car insurance policy and goes into effect when the person at fault for your accident has insurance, but their policy limits are not high enough to fully compensate you for your loss.
For example, you are in an accident, which was not your fault. You have losses totaling $45,000, but the other driver's insurance only covers $25,000 in losses. If you carry UIM, which in South Carolina you are required to have at least $25,000 in coverage, the remaining balance will then be entered as a claim agaisnt your UIM coverage. This coverage will hopefully be enough to compensate the remainder of your losses.