CV[p, i] is a 3-dimensional Cartesian vector and is transformed into CartesianPair[CartesianMomentum[p], CartesianIndex[i]] by FeynCalcInternal.
Overview, FV, Pair, CartesianPair.
CV[p, i]\overline{p}^i
CV[p - q, i]\left(\overline{p}-\overline{q}\right)^i
FCI[CV[p, i]] // StandardForm
(*CartesianPair[CartesianIndex[i], CartesianMomentum[p]]*)ExpandScalarProduct is used to expand momenta in CV
ExpandScalarProduct[CV[p - q, i]]\overline{p}^i-\overline{q}^i