Final answer:
The label 'Dangerous when wet' refers to substances that emit flammable gases or may violently react when in contact with water, and it is indicated in the white section of the NFPA 704 Hazard Identification System's diamond.
Step-by-step explanation:
The hazard label "Dangerous when wet" refers to substances that pose a risk of dangerous reactions upon contact with water. This would generally include substances that are part of the category "articles, which emit flammable gases by interaction with water", as well as other materials that may react with water to create a fire or explosion hazard. An example of such a substance would be metals like sodium or potassium, which can react violently when they come in contact with water.
The National Fire Protection Agency (NFPA) 704 Hazard Identification System is used to provide information about the hazards of these substances. The white section of the NFPA's hazard diamond indicates special hazards, including whether a chemical reacts dangerously with water. These substances are considered characteristic hazardous wastes, which the US Environmental Protection Agency (US EPA) notes can pose a substantial present or potential hazard to human health or the environment if not managed properly.