What does the four-digit number on a placard or orange panel indicate?

The four-digit number is the UN (United Nations) or NA (North American) identification number assigned to that specific hazardous material. For example, UN1203 is gasoline, UN1075 is liquefied petroleum gas, UN1993 is flammable liquid n.o.s. (not otherwise specified). The number lets emergency responders look up the exact material and its properties in the Emergency Response Guidebook (ERG) — the orange book carried in every fire and ambulance crew. The number is displayed on the placard itself or on an orange rectangular panel adjacent to the placard. The four-digit number is far more useful in an emergency than the hazard class alone because there are thousands of materials in each class with different properties.