S.U.M. means supplemental uninsured or underinsured motorist coverage. When you buy auto insurance in New York, you are required to protect the driving public against your causing an auto accident - that is what your liability limits are for, the other driver.
What most people are NOT aware of is S.U.M. limits are to protect you against other at-fault drivers, who carry either little or no insurance (it lapsed?). So this is a way to protect you & your family against other drivers carrying low limits of insurance. It is a way for you to collect on your own insurance policy.
What drivers me crazy is when I see prospects or auto insurance policies that have the minimum NY State limits of $25,000 per person/ $50,000 per accident for S.U.M. Mr or Mrs. Insurance broker, why are you protecting them with high liability limits for other drivers and not themselves? You should always, always match your S.U.M. limits to your liability limits or your insurance person isn't doing their job. In reality, that is how you can judge if they have your best interest in mind or their own loss ratio (thus bonus money).