CVE[p, i] is a D-4-dimensional Cartesian vector and is
transformed into
CartesianPair[CartesianMomentum[p,D-4], CartesianIndex[i,D-4]]
by FeynCalcInternal.
Overview, FVE, Pair, CartesianPair.
CVE[p, i]\hat{p}^i
CVE[p - q, i]\left(\hat{p}-\hat{q}\right)^i
FCI[CVE[p, i]] // StandardForm
(*CartesianPair[CartesianIndex[i, -4 + D], CartesianMomentum[p, -4 + D]]*)ExpandScalarProduct is used to expand momenta in
CVE
ExpandScalarProduct[CVE[p - q, i]]\hat{p}^i-\hat{q}^i