What's the difference between Uninsured and Underinsured motorist insurance?

Question

In the US, one can often add Uninsured and Underinsured Motorist coverage independently to one's car insurance. What exactly is the difference? It seems that, going by the name only, Underinsured should cover a strict subset of Uninsured since every person that is uninsured is also underinsured but not vice versa. The fact that one can add these coverages independenly suggests that there is more to that. Specifically, what is covered/not covered in the following scenarios:

1. add only Uninsured
2. add only Underinsured

Answer 1

You are correct that uninsured covers you if the at fault driver doesn't have insurance; and under-insured covers you if the costs to your vehicle or medical are above the limits of the other driver. Without this under-insured coverage you would have to sue to get the additional money from the other driver.

The required minimum coverage for auto insurance is set by the state, but because the minimum limits vary by state, you can be hit by a driver with the required amount of insurance for their state yet it be lower than the requirements for your state.

Your state can require that you purchase uninsured and/or under-insured coverage.

Answer 2

The definitions might be different depending on the provider, but here's eSurance's definition, which seems reasonable to me:

The difference between uninsured and underinsured coverage

Uninsured motorist coverage protects you if you're in an accident with an at-fault driver who doesn't carry liability insurance.

Underinsured motorist coverage, on the other hand, steps in when you're in an accident with an at-fault driver whose liability limits are too low to cover the damage or medical expenses. The at-fault driver's insurance will typically pay for all damages up to the policy limits, and then your underinsured motorist coverage will cover the excess amount up to the limits you select.

Adding underinsured only would NOT cover you if you had an accident with someone who has NO insurance. In other words, uninsured is NOT a subset of underinsured - they are orthogonal.