It isn't a setting but rather an action.  Information about backups is kept
in the volume history file.  Periodically, one deletes records from this
file to clear up database backups.  The command is delete volhistory
type=dbbackup todate=today-x where x is the number of copies of the
database backup you want to keep.

For the group, isn't it interesting how these questions run in circles?

Kelly