CouplingConstant is an option for several Feynman rule functions and for CovariantD and FieldStrength. In the convention of the add-on PHI, CouplingConstant is also the head of coupling constants. CouplingConstant takes three extra optional arguments, with heads RenormalizationState, RenormalizationScheme and ExpansionState respectively. E.g. CouplingConstant[QED[1]] is the unit charge, CouplingConstant[ChPT2[4],1] is the first of the coupling constants of the Lagrangian ChPT2[4].
CouplingConstant[a_,b_,c___][i_] := CouplingConstant[a,b,RenormalizationState[i],c].
Overview, CovariantD, FieldStrength.