This printer language command is used at the beginning and end of every printer data stream. Usually it is already in the data stream, especially printing via Windows or SAP.
It is used to separate data streams. It is for example definitively needed for the stapling commands.
So every PCL5 print job should begin with:
@PJL ENTER LANGUAGE=PCL
and ends with:
If this is not the case, it is strongly recommended to use the ELP_Command=ET; This wraps the job into UEL statements and provides a well formed PJL header.