To do back translation in two directions (e.g., X⇒Z and Z⇒X), we need monolingual data from both sides (e.g., X and Z), however, in TA-NMT, the monolingual data of Z is not necessary.
in the translation of X⇒Z or Y⇒Z, Back-Trans uses additional monolingual data of Z while TA-NMT does not, that is why BackTrans outperforms TA-NMT in these directions.