The numbered placards are using an ID system, there is a reference book which correlates the number to a specific chemical/material.

The NFPA/diamond system uses the 1-4 numbering to indicate the relative safety factor of what's in the truck or in a building. That's a very simple explanation, it's pretty easy to find out more, do a google search.

You can obtain a copy of the guide book with all of the specific material codes at truck stops and thru bookstores. I'd probably do a google search for that too. I got mine at a truck stop in a nice small size that I keep in my vehicle.