SUNDelta[a, b] is the Kronecker-delta for SU(N) with color indices a and b.
See also: ExplicitSUNIndex, SD, SUNF, SUNIndex, SUNSimplify, Trick.
Symbolic arguments to SUNDelta are transformed into the data type SUNIndex and integer arguments are transformed to ExplicitSUNIndex. The difference is that SUNSimplify will only sum over symbolic indices.
The FeynCalc Book | SUND | SUNF |