Copied in from this post What do these letters/numbers stand for then? - THE UK PROFESSIONAL DRIVERS FORUM (INTERACTIVE) - Trucknet UK
Cruise Control:
in laymans terms please
cheers.
Hi Cruise Control, That’s a good question mate. 
As you can see, the Hazard Warning Panel is split into 5 parts.
Top left (orange) This part contains a code (Emergency Action Code) to tell firefighters what they can safely squirt (water, foam or dry powder) at the substance being carried in the event of fire or spillage.
Middle left (orange) This part contains a four-digit number assigned by the UN, which identifies the substance being carried.
Bottom left (orange) This part contains a phone number where expert advice (usually an industrial chemist) on the substance being carried can be obtained in English at any time during carriage.
Top right (white) This part contains a placard to indicate the danger presented by the substance being carried. If the substance being carried has more than one hazard, then extra placards are added alongside the one in the pic. (Max placards = 3 in total.)
(There are 9 classes of dangerous goods, the others are shown in the ADR ‘sticky’ in the Safety and Law forum)
Bottom right (orange) This part is a space reserved for the company to put their logo, but it may be left blank if the company don’t wish to put their logo on the hazard warning panel.
The exact meaning of the coded info on hazard warning panel you saw is:
Top left (orange):
“3” = Fight fire with foam
“Y” = A spillage should be contained + Danger of violent reaction with water + Use (wear) a BA set when fighting fire, + Use (wear) Fire-Kit ( = chemical suit not necessary)
Middle left (orange): “1223” = Kerosene (paraffin) [but may also be carrying Diesel fuel]
I’ve assembled pictures of the placards for all 9 UN Classes of dangerous goods in my answer to Hutchie about half way down the page in 
THIS POST
I hope that helps.