The difference between underinsured motorists (UIM) and uninsured motorists (UM) coverage is that UIM applies when the at-fault driver:

解析

Underinsured motorists coverage applies when the at-fault driver does carry liability insurance, but the limits are insufficient to fully pay the injured insured's damages; UIM makes up part of the shortfall. Uninsured motorists coverage applies when the at-fault driver has no liability insurance or cannot be identified (such as a hit-and-run). Both protect the innocent insured from another driver's inadequate coverage.