ImplicitPauliIndex is a data type. It mainly applies to names of quantum fields specifying that the corresponding field carries an implicit Pauli index.
This information can be supplied e.g. via DataType[QuarkFieldChi, ImplicitPauliIndex] = True, where QuarkFieldChi is a possible name of the relevant field.
The ImplicitDiracIndex property becomes relevant when simplifying noncommutative products involving QuantumFields via ExpandPartialD, DotSimplify.
Overview, DataType, ImplicitSUNFIndex, ImplicitDiracIndex
Default (possibly unwanted) behavior
ex = QuantumField[QuarkFieldChiDagger] . CSI[i] . QuantumField[QuarkFieldChi]\chi ^{\dagger }.\overline{\sigma }^i.\chi
ExpandPartialD[ex]\overline{\sigma }^i.\chi ^{\dagger }.\chi
Now we let FeynCalc know that QuarkFieldChiDagger and QuarkFieldChi carry an implicit Pauli index that connects them to the Pauli matrix.
DataType[QuarkFieldChi, ImplicitPauliIndex] = True;
DataType[QuarkFieldChiDagger, ImplicitPauliIndex] = True;ExpandPartialD[ex]\chi ^{\dagger }.\overline{\sigma }^i.\chi