You may think so, but if you are like most people I come into contact with, you do not even know what it is. "UIM" is underinsured motorist coverage. Do not confuse UIM with the required UM. "UM" is uninsured motorist coverage that you can only use if the at fault driver has no liability insurance.
UIM is totally optional bodily injury car insurance you buy to protect yourself, your family, and those in your car when an at fault driver does not have enough liability coverage to pay for the harm that they have caused. Please call your car insurance agent and ask him or her to slowly and completely explain UIM, including potential stacking. If you still have questions, feel free to call me. Be safe, and look to avoid car accidents.
If you are involved in an accident, the minimum limit of liability insurance is $25,000. In even a minor accident, you can go through that amount pretty quickly with medical care only.