Small Arms Defense Journal has a thorough article on the treaty here.

The advertised purpose of the ATT was to require countries who become parties to adopt meaningful and hopefully effective national laws, based on agreed international standards, for regulating the export and import of conventional arms such as tanks, combat aircraft, attack helicopters, artillery, combatant naval vessels, and, presumably, small arms and light weapons.  Realistically, 99% of the energy and rhetoric concerning the treaty has always been about small arms, not larger weapons systems.  It’s also instructive that only about 25% of the world’s countries currently have even rudimentary regulations covering the topic.