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Placards can be used to communicate two types of hazards — primary and subsidiary:
Anyone who is responsible for placarding a transport vehicle, freight container, portable tank, unit load device, or rail car must follow the subsidiary placarding requirements.
Generally, only the primary placards are displayed. However, 172.505 of the hazardous materials regulations requires additional placards to be displayed under certain conditions for several subsidiary hazards. The following subsidiary placards must be displayed:
Other subsidiary placards are permitted to be displayed but are not required.
Subsidiary placards, just like primary placards, must have the hazard class or division number in the lower corner of the placard.