ImplicitSUNFIndex is a data type. It mainly applies to names of quantum fields specifying that the corresponding field carries an implicit SU(N) index in the fundamental representation.
This information can be supplied e.g. via DataType[QuarkField, ImplicitSUNFIndex] = True, where QuarkField is a possible name of the relevant field.
The ImplicitSUNFIndex property becomes relevant when simplifying noncommutative products involving QuantumFields via ExpandPartialD, DotSimplify.
Overview, DataType, ImplicitDiracIndex, ImplicitPauliIndex
Default (possibly unwanted) behavior
ex = QuantumField[AntiQuarkField] . SUNT[a] . QuantumField[QuarkField]\bar{\psi }.T^a.\psi
DotSimplify[ex]\bar{\psi }.\psi T^a
ExpandPartialD[ex]\bar{\psi }.\psi T^a
Now we let FeynCalc know that AntiQuarkField and QuarkField carry an implicit color index that connects them to the color matrix.
DataType[QuarkField, ImplicitSUNFIndex] = True;
DataType[AntiQuarkField, ImplicitSUNFIndex] = True;DotSimplify[ex]\bar{\psi }.T^a.\psi
ExpandPartialD[ex]\bar{\psi }.T^a.\psi