In page 18 is explained that category can be used by the Admin to determine the nature of dice rolls. While this part wasn't changed on the 1.1 update (It only talks about the exorcists and their powers), I asume that it also extends to the weapons, so higher CAT weapons would be more destructive/powerful (as a house rule, with the integration of category on weapons I'm also asking my players to choose/describe their weapons according to their CAT, so as they are on CAT 2 now, they still have no access to highly destructive weapons), and maybe even make certain actions don't even require rolls.
An example I'm thinking is using like a CAT 4 weapon against mercenaries or police forces (which are normal humans), I wouldn't even ask for the roll then, they just kill/obliterate them.