4 sides require placards. Front, back, and both left/right sides.
It is a DOT hazardous materials placard indicating compressed oxygen.
The placard on a tanker carrying hazardous materials will tell you the class of the hazard (flammable, explosive, corrosive, etc). If you have access to the North American Guide or a similar reference, the placard will also tell you something a bit more specific about the identity of the hazardous material - its category, or sometimes its chemical name.
The hazard class of the substance being carried
Shipment contains a mixed load of hazardous materials
1D
Hazardous materials identified in vehicles and rail cars through the use of placards.
The DOT hazardous materials placard for organic peroxides may be - an all-yellow diamond shape with "ORGANIC PEROXIDE" and "5.2" lettering, or - the revised DOT placard which is red on top and yellow on the bottom, with a stylized flame in the red area.
Placards are not used in maritime shipping of hazardous materials. Even in US ground transportation, placards are not required for the shipment of "Limited Quantities."
The DOT placard for organic peroxides is a yellow diamond with "ORGANIC PEROXIDE" lettering and the hazard identifying number "5.2" on it. The revised international version is red on top, yellow on the bottom, with a stylized flame symbol in the red and the numbers 5.2 in the yellow.
Front
A placard identifying hazard class 6 describes the presence of a poisonous gas. For an image, see the related link, below.
self heating or pyrophoric materials