What does "UN" stand for in hazardous materials labeling?

In hazardous materials labeling, "UN" stands for United Nations. The United Nations created a system of classification and identification for hazardous substances to ensure safety in transportation and handling. This system includes a numbering system wherein hazardous materials are assigned unique UN numbers. These numbers help transporters, first responders, and others quickly identify the nature of the materials involved, enhancing communication about risks and safety measures.

The other options do not accurately capture the significance of "UN" within the context of hazardous materials. Universal Number, Unrestricted Nuance, and Undefined Notation do not relate to recognized systems for classifying hazardous materials under international guidelines. Understanding that "UN" represents the United Nations is crucial for following regulations and ensuring the safe transport of hazardous substances.