A Class 8 corrosive liquid is assigned Packing Group I, while another Class 8 corrosive liquid is assigned Packing Group III. What does this difference tell you?

49 CFR 171.8 (definitions — packing group)

Two loads can share the same placard and hazard class, yet behave completely differently in a spill depending on packing group. Recognizing PG I as the most severe tier explains why your paperwork, packaging, and penalty exposure change even when the placard looks identical.