enque/backup_file is a SAP Parameter attribute which is used to control Name (full path) of the backup file information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Full path to backup file. This is used to enter the locks, which are
transferred to the update, in the lock table again after the enqueue
server is restarted.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.