What is the difference between a UN number and a Kemler number

A UN number identifies the dangerous substance — the 4-digit code from the Orange Book classifying the substance across all transport modes. A Kemler number is a 2- or 3-digit hazard identification code on the upper section of the ADR vehicle orange plate, indicating the primary hazard and any subsidiary hazard of the substance on board. The first digit represents the primary hazard class; a repeated digit signals an intensified hazard — 33 indicates a highly flammable liquid, 88 indicates a highly corrosive substance. The UN number occupies the lower section of the same plate. Both codes appear together on every ADR vehicle carrying dangerous goods in bulk, but they originate from different parts of the ADR regulatory framework and answer different questions for emergency responders at an incident.