It indicates a flammable or explosive gas.
explosives
symbol, UN hazard class number, color, UN identification number
COLOR
The Dangerous When Wet placard is a square on point (or diamond) placard with white markings on a blue background. The words "Dangerous When Wet are printed across the middle, with a drawing of a flame centered in the upper corner, and the numeral four (4) in the bottom corner to indicate the Hazard Class.
The hazard class of the substance being carried
The placard for propane is Class 2.1. It is a diamond-shaped placard with a red upper half and a lower half that is predominantly white, displaying the Class number and the UN identification number for propane.
A placard identifying hazard class 6 describes the presence of a poisonous gas. For an image, see the related link, below.
Poisonous or infectious materials are in Hazard Class 6. The several labels and placards for Class 6 are white in color, with black printing.
Bottom
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.
Hazard Class 3
Class 1 materials are explosives.Materials that have an explosive hazard are in Hazard Class 1.