KD[i, j]
is the Kronecker delta in dimensions.
[i, j] KD
[KD[i, j] KD[i, j]] Contract
[a, b] // StandardForm
KD
(*KD[a, b]*)
[KD[a, b]] // StandardForm
FCI
(*CartesianPair[CartesianIndex[a], CartesianIndex[b]]*)
[FCI[KD[a, b]]] // StandardForm
FCE
(*KD[a, b]*)