This is like uninsured motorist coverage, but it applies when the other driver is not completely uninsured, but when the other driver’s insurance policy limit is insufficient to cover all the injuries he caused.  One driver cannot be both uninsured and underinsured, but if you are a passenger in a vehicle and the driver of the vehicle you are in and another driver are both at fault for your injuries, one if one is uninsured and the other underinsured, you may have both claims against your own policy.