Under what condition does the DOT require hazardous contents to have a four-digit number?

The Department of Transportation (DOT) mandates that hazardous materials display a four-digit number when they are moving in interstate commerce. This system is part of the Hazardous Materials Transportation Act, which aims to ensure safe and secure transport of dangerous goods across state lines. The four-digit number corresponds to the United Nations (UN) or North American (NA) identification numbers that categorize the material, providing critical information to emergency responders and transport personnel regarding the nature and potential hazards of the materials being transported. This identification increases safety during transit and ensures compliance with regulations that govern the shipment of hazardous substances on a national level. In contrast, other scenarios like local transit within a state, storage in warehouses, or display at a facility do not have the same requirements for a four-digit identification, as they fall under different regulations or do not involve interstate transport where hazardous materials might pose broader risks.