This symmetry focuses on changing the charge of a given particle to its opposite, and thus changing the particle into its mirror image- however, this results in a change inside of the Weak Force, and it no longer obeys symmetry. QED can't violate this symmetry, whereas the Strong Force can, but doesn't for some unknown reason.

The two forces are the Weak Nuclear Force and the Electromagnetic Force.

Gauge symmetry was generalized in 1954 by C.N. Yang, and Mills, in what is called Yang-Mills theory, which found its application in QFT, specifically the Weak Force.