Tier 1 weapons refer to any weapon that can be held and fired by a non-GE human. Tier 2 weapons were originally anything too large to be shot by a person, but with strength boosting that came to include a lot of weapons that couldn’t reasonably be associated with Tier 1. The “naught-point” tiers were developed to describe the range of weapons that could be fired by humans of various GE levels. (source: GCSC FAQ hawkingnet site)
weapons can be carried and fired by exceptionally strong non-GE humans, and fire one round at a time.
weapons can be carried and fired by mildly GE humans, and fire one XL caliber round or two normal caliber rounds at a time.
weapons can be carried and fired by moderately GE humans and the majority of benders, and usually fire two or three small to large caliber rounds at a time.
weapons can be carried and fired by only the most grossly modified GE humans, and some of the stronger benders, and can fire up to four XL caliber rounds simultaneously at muzzle velocities approaching the escape velocity of Earth.
weapons that are too large or too energetic to be carried and fired by any known GE humans. Some exceptionally strong benders can actually use the smallest of these weapons.
Not an official weapon tier, but it’s become fairly common to refer to non-lethal weapons as naught-point-one, though most are technically Tier 1 by the GCSC classification tables, while others aren’t even considered weapons.