SUNNToCACF is an option of SUNSimplify and CalcColorFactor. If set to True, the Casimir operator eigenvalues CA (=n_c) and CF (=(n_c^2-1)/(2 n_c)) are introduced.
Overview, CalcColorFactor, SUNSimplify, Trick, SUNN, CA, CF.
SUNSimplify[SUNDelta[SUNIndex[a], SUNIndex[a]], SUNNToCACF -> True]2 C_A C_F