CSI[i] can be used as input for 3-dimensional \sigma ^i with 3-dimensional Cartesian index i and is transformed into PauliSigma[CartesianIndex[i]] by FeynCalcInternal.
CSI[i]\overline{\sigma }^i
CSI[i, j] - CSI[j, i]\overline{\sigma }^i.\overline{\sigma }^j-\overline{\sigma }^j.\overline{\sigma }^i
StandardForm[FCI[CSI[i]]]
(*PauliSigma[CartesianIndex[i]]*)CSI[i, j, k, l]\overline{\sigma }^i.\overline{\sigma }^j.\overline{\sigma }^k.\overline{\sigma }^l
StandardForm[CSI[i, j, k, l]]
(*CSI[i] . CSI[j] . CSI[k] . CSI[l]*)