 The quality of the backward system - which is trained on the available parallel data and used for the back-translation - has been shown in many studies to affect the performance of the final NMT model.
<clarity>  The quality of the backward system - which is trained on the available parallel data and used for the back-translation - has been shown in many studies to affect the performance of the final NMT model.
 The target-side side monolingual data has been used in the back-translation - has been shown in many studies to affect the performance of the final NMT model.