You need a UCR if you are operating a commercial vehicle and intend to transport goods or passengers across state lines. This applies only to vehicles whose gross vehicle weight (GVW) or gross vehicle weight rating (GVWR) exceeds 10,000 pounds, vehicles carrying 10 or more passengers (including the driver), and vehicles hauling hazardous materials requiring placarding. If your passengers or goods have crossed state lines prior to you picking it up, or after delivery, you will also need to register for UCR. A common example of this is if you transport passengers to and from an airport, transport goods to a shipping facility who will distribute it nationwide, etc. If you do not meet any of these criteria, you are not required to register.
Trucking companies should register all applicable vehicles in their fleet, but brokers and freight forwarders organizing interstate commerce will also need to complete the registration in the single vehicle tier. If your vehicles are not crossing any state lines, you are not required to register.