The general rule is going to be: If in bulk, you always need a placard. If non-bulk, then it depends on if the hazard class is in Table 1 or 2, and the amount that is being shipped. Also, in most cases, 4 placards are required, one on each side and one on each end.

What three things do you need to know to decide which placards if any you need?

You can decide which placards to use if you know these three things: Material's hazard class. amount being shipped. amount of all hazardous materials of all classes on your vehicle.

What is a 6.1 placard?

Hazard Class 6.1 – Toxic Substances, Permanent Self-Stick Vinyl, Non-Worded Placard.