PartialDRelations is an option for ExpandPartialD. It is a list of rules applied by ExpandPartialD at the end.
Overview, FCPartialD, ExpandPartialD, LeftPartialD, LeftRightPartialD, RightPartialD.
QuantumField[A, {\[Mu]}] . QuantumField[B, {\[Mu]}] . LeftPartialD[\[Nu]]
ExpandPartialD[%, PartialDRelations -> {A -> C}]A_{\mu }.B_{\mu }.\overleftarrow{\partial }_{\nu }
C_{\mu }.\left(\left.(\partial _{\nu }B_{\mu }\right)\right)+\left(\left.(\partial _{\nu }C_{\mu }\right)\right).B_{\mu }