AuxiliaryMomenta is an option of FCLoopPropagatorsToLineMomenta, FCLoopIntegralToGraph and other functions. It specifies auxiliary momenta that do not correspond to external legs, i.e. don’t really flow through the lines, e.g. n and \bar{n} in SCET or v in HQET.
Overview, FCLoopPropagatorsToLineMomenta, FCLoopIntegralToGraph.