The string value of $TypesettingDimE determines which symbols will be displayed above (D-4)-dimensional momenta, Dirac matrices, metric tensors and polarization vectors. This concerns only typesetting in the TraditionalForm output and doesn’t change the physical behavior of those objects.
Overview, $TypesettingDim4, $TypesettingDimD.
$TypesettingDimE{}^{\wedge}