What is the difference between a packing group and a hazard class in ADR?

A hazard class identifies the category of danger a substance presents — flammable, corrosive, toxic, radioactive, or explosive. The packing group ranks the intensity of that danger within the class. Petrol (Class 3, PG II) and diethyl ether (Class 3, PG I) share the same hazard class but sit at different packing groups because diethyl ether has a lower initial boiling point and presents a higher danger level within Class 3. Both the hazard class and the packing group appear as separate mandatory fields on the ADR transport document and on package labels. One identifies what type of danger the substance presents; the other states how severe that danger is.