$BreitMaison is a legacy switch for the Breitenlohner-Maison-t’Hooft-Veltman scheme.
Use FCSetDiracGammaScheme to specify a scheme for handling Dirac matrices in dimensional regularization and FCGetDiracGammaScheme to check the current setting.
Overview, FCSetDiracGammaScheme, FCGetDiracGammaScheme.