SD[i, j] denotes the SU(N) Kronecker delta with color indices i and j in the adjoint representation.
SD[i,j] is transformed into SUNDelta[SUNIndex[i],SUNIndex[j]] by FeynCalcInternal.
SD[a, b]\delta ^{ab}
SD[a, b] // FCI // StandardForm
(*SUNDelta[SUNIndex[a], SUNIndex[b]]*)SD[a, b] // FCE // StandardForm
(*SD[a, b]*)