SUNFIndex[a] is an SU(N) index in the fundamental
representation. If the argument is an integer, SUNFIndex[a]
turns into ExplicitSUNFIndex[a].
SUNFIndex[i]i
SUNFIndex[i] // StandardForm
(*SUNFIndex[i]*)SUNFIndex[2]2
SUNFIndex[2] // StandardForm
(*ExplicitSUNFIndex[2]*)SUNFDelta[i, j] // FCI // StandardForm
(*SUNFDelta[SUNFIndex[i], SUNFIndex[j]]*)