Actually there are three categories: 1, 2 & 3. The two columns listed for each category, (1 & 2), are simply the conversion from TBq to Curies, (1 TBq = 27.27 Curies). They aren't ranges; they are threshold values for that category, (at, or above). Category 3, a US NRC thingy, is established at 10% of cat 2 threshold. The reasoning, the more you glow the more they want to know! Tracability and accountability!