What does a Kemler hazard identification number with duplicate digits (such as '33' or '66') on an orange hazmat placard signify?

1. An intensification or heightened severity of the primary hazard (e.g. 33 = highly flammable liquid)
2. That the truck is carrying two identical driver assistants
3. That the transport route is 33 kilometres long
4. That the fuel tank has a 33% discount on road tax