Halon gas has been used since 1950 as a fire control agent in rooms which contain electronic equipment. It prevents combustion, is safe for humans, and doesn't interfere with electronics. However, it is horrible for the ozone, 16 times worse than CFC's. Halon is also very expensive. Since 1993, due to the Montreal agreement signed by 150 countries, the gas has been banned for manufacture and use, and by December of 2003, all buildings which still use it will have to cease. This button probably aborted a Halon release in case of accidental alarm.