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.