 The target-side side monolingual data has been used in the back-translation approach to improve the forward (target) translation model.
<meaning-changed>  The target-side side monolingual data has been used in the back-translation approach to improve the forward (target) translation model.
 The quality of the backward system - which is trained on the available parallel data and used for the back-translation approach to improve the forward (target) translation model.