HazMat Regulations, Definitions, and Communication Rules
The regulatory frame
Hazardous materials transportation in the United States is governed by Title 49 of the Code of Federal Regulations, primarily Parts 171 through 180. Part 171 covers general rules and definitions, Part 172 covers the communication standards (shipping papers, marking, labeling, placarding, and emergency response information), and Parts 173, 177, 178, and 180 cover packaging, motor carriage, package specifications, and continuing qualification of packagings. California adopts the federal rules and adds state requirements through the California Vehicle Code and Title 13 of the California Code of Regulations, so a driver must satisfy both layers.
Hazard classes and divisions
There are nine hazard classes. Class 1 is explosives and is broken into divisions 1.1 through 1.6 based on the character of the blast and fire hazard. Class 2 is gases, split into 2.1 flammable, 2.2 non-flammable/non-toxic, and 2.3 toxic. Class 3 is flammable and combustible liquids. Class 4 covers flammable solids (4.1), spontaneously combustible materials (4.2), and dangerous when wet materials (4.3). Class 5 is oxidizers (5.1) and organic peroxides (5.2). Class 6 is toxic substances (6.1) and infectious substances (6.2). Class 7 is radioactive material, Class 8 is corrosives, and Class 9 is miscellaneous hazardous material.
The communication package
Every shipment requires a consistent set of communications: shipping papers, package marking, package labels, placards on the vehicle, and emergency response information. The traps in questions usually involve one element being present while another is missing, or a driver "fixing" a paperwork problem by making a change that is not permitted.
Rule of thumb
If you can name the hazard class, division, and packing group from the shipping paper, you can predict the label and placard. When a question describes a mismatch between paper and placard, the answer is almost always to stop and resolve it before moving, not to proceed and correct it later.