Under ADR dangerous goods regulations, what does a doubling of a digit in the top half of an orange hazard plate represent (e.g., hazard code '33')?

1. The hazard is minor and requires no action
2. An intensification of that particular hazard (e.g. '33' indicates a highly flammable liquid)
3. The vehicle is carrying two identical chemical tanks
4. The chemical has a boiling point of 33 degrees Celsius