A hazmat shipping paper lists a material's identification number as 'UN1203.' What does this number represent, and where should a driver look for emergency guidance tied to it?

49 CFR 172.101 Hazardous Materials Table (identification numbers); AAMVA CDL Manual (nhcdm.pdf) Section 2.23; Emergency Response Guidebook (ERG), introductory instructions

In a spill or crash, the ID number on the shipping papers or placard is the fastest way to find out what is actually dangerous about the material and how to respond safely. Drivers, even without a hazmat endorsement, benefit from knowing this system exists so they know what information to give responders if they're near an incident.