Non-stacking UM/UI policies ensure you have at least $25,000 (or any other amount over $25,000) in bodily injury or property damage liability coverage. If the at-fault driver had at least that amount, your UM/UI policy will not apply even if your damages exceed the other driver’s policy limits.
Stacking UM/UI policies give you extra coverage on top of whatever the at-fault motorist has, so you get the amount of your policy after their insurance has paid to its limit. These policies may have more expensive premiums, but they provide more coverage in an accident with an underinsured driver.