VirtualBoson is an option for
PolarizationSum and DoPolarizationSums. If set
to True, FeynCalc will not complain when you apply the gauge trick
(i.e. replace the polarization sum by - g^{\mu
\nu} for a particle that is not massless. This is useful when
computing processes that involve a virtual photon as an external
state.
Overview, PolarizationSum, DoPolarizationSums.