What does the doubling of a digit in the Hazard Identification Number signify (for example, '33' or '88' or '66')?

1. A clerical printing error on the orange placard
2. An intensification of that specific primary hazard (e.g., 30 = flammable liquid, 33 = highly flammable liquid)
3. The vehicle is carrying two identical separate tanks
4. The cargo must be kept at a temperature below zero degrees