The ones that SAY what they are are simple to understand.

So what are those removable square number placards that you see on trucks (esp tankers) that say something like 1634 (I just made up that number)? It's obviously some kind of identification code for contents, but there's nothing with it to identify what it is. Ammonia? Chlorine? Ammonium Perchlorate?

Or is it the specific collection of hazards shown in the poster: 1) Explosive + 6) Poison + 2) Combustible + 4) Dangerous ?