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 reconstructed from
the result in terms of n_c using
heuristics. The reconstruction is not always perfect, but mostly
sufficient at tree and one-loop level.
Overview, SUNSimplify, SUNN, CA, CF.
SUNSimplify[SUNDelta[SUNIndex[a], SUNIndex[a]]]2 C_A C_F
SUNSimplify[SUNDelta[SUNIndex[a], SUNIndex[a]], SUNNToCACF -> False]N^2-1