Hazard Classes are classified by Pictograms and Categories, depending on the type and severity of a Hazard, a class may have more then one category in it.
Category 1 = greatest level of hazard
Category 2 = second greatest level of hazard
Category 3 = least hazardous
These categories may sometimes be broken down into sub categories, for example 2A and 2B, in this case 2A would be more hazardous then 2B.