As already mentioned, the Apertium structural transfer detects sequences of lexical forms which need to be translated together to prevent them from being translated word for word, which would result in an incorrect translation.
adding to the phrase table of a PBSMT system all the bilingual phrase pairs which either match one of these sequences of lexical forms in the structural transfer or an entry in the bilingual dictionary ensures that all the linguistic information of Apertium is encoded with the minimum amount of phrase pairs.