CSIS[p]can be used as input for 3-dimensional \sigma ^i p^i with 3-dimensional Cartesian vector p and is transformed into PauliSigma[CartesianMomentum[p]] by FeynCalcInternal.
CSIS[p]\overline{\sigma }\cdot \overline{p}
CSIS[p] // FCI // StandardForm
(*PauliSigma[CartesianMomentum[p]]*)CSIS[p, q, r, s]\left(\overline{\sigma }\cdot \overline{p}\right).\left(\overline{\sigma }\cdot \overline{q}\right).\left(\overline{\sigma }\cdot \overline{r}\right).\left(\overline{\sigma }\cdot \overline{s}\right)
CSIS[p, q, r, s] // StandardForm
(*CSIS[p] . CSIS[q] . CSIS[r] . CSIS[s]*)