$Larin is a legacy switch for the Larin-Gorishny-Atkyampo-DelBurgo scheme. The modern way is to use FCSetDiracGammaScheme to specify a scheme for handling Dirac matrices in dimensional regularization and FCGetDiracGammaScheme to check the current setting.
Overview, FCSetDiracGammaScheme, FCGetDiracGammaScheme.