The string value of $TypesettingDimD determines which
symbols will be displayed above D-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, $TypesettingDimE.
$TypesettingDimD\text{}