The process described is an equilibrium between two equivalents of an alkyl or aryl magnesium halide on the left of the equation and on the right side, one equivalent of the dialkyl or diaryl magnesium compound and magnesium halide salt.
This process describes a state where there are equal amounts of two types of organomagnesium compounds (alkyl or aryl magnesium halides) on both sides of an equation, with one equivalent of a dialkyl or diaryl magnesium compound and a magnesium halide salt on the right side.