What primary information is provided by the "four digit" UN number on a placard?

The "four digit" UN number on a placard serves the primary purpose of identifying a specific hazardous material. Each UN number is unique and corresponds to a specific substance or a group of substances, which allows first responders and others to quickly recognize the type of hazardous material they are dealing with. This identification is crucial for ensuring the appropriate response actions are taken, as different substances may require different handling, containment, and mitigation strategies to address potential risks effectively.

While the other choices refer to important information that might be needed during a hazardous materials incident, they do not capture the primary function of the UN number, which is solely to provide an unmistakable identification of the hazardous material involved.