'User Archive' is explained very clearly in NBU Admin Guide I.
Please find links to your NBU version in the 'Handy Links' in my signature.
Extract from p. 631 of NBU 7.1 Admin Guide I:
A user initiates a user archive through the Backup, Archive, and Restore client interface.
A user archive backup first backs up the files that the user indicates. Then it deletes the
files from the local disk if the backup is successful. Archive backups free local disk space
while retaining a copy for future use. The copy is kept until the retention period expires.
Users can start archives only during the times that are specified in the schedule Start
Window tab.