DOT Hazard Classification System
The hazard class of dangerous goods/commodities is indicated either by its class (or division) number or name. Placards are used to identify the class or division of a material. The hazard class or division number must be displayed in the lower corner of a placard and is required for both primary and subsidiary hazard classes and divisions, if applicable.
First responders in the U.S. are trained to help identify a hazardous material during the first 15 minutes of an incident, based on the U.S. DOT/Pipeline and Hazardous Materials Safety Administration (PHMSA) Emergency Response Guidebook. The following is from Page 6.