ExplicitPauliIndex[ind] is an explicit Pauli index,
i.e., ind is an integer.
Overview, PauliChain, PCHN, PauliIndex, PauliIndexDelta, PIDelta, PauliChainJoin, PauliChainCombine, PauliChainExpand, PauliChainFactor.
PCHN[SI[i], 1, 2]\left(\bar{\sigma }^i\right){}_{12}
PCHN[SI[i], 1, 2] // FCI // StandardForm
(*PauliChain[PauliSigma[LorentzIndex[i]], ExplicitPauliIndex[1], ExplicitPauliIndex[2]]*)