Yes, good information provided by Matti. You really don't need "BACKUP_OPTIONS" unless there is a nfs version change between rear server (backup) and client end systems. You may consider adding "BACKUP_PROG_EXCLUDE" to exclude un-wanted data such as /var/crash, /media which may not be required which otherwise endup in creating a huge backup image dump.
The command "rear -d -v mkbackup" when run after configuring rear backup file "/etc/rear/local.conf" with correct parameters would generate necessary files and copy files over NFS to the destination rear backup server. There would be a directory created with hostname on the remote rear backup server which would show up a file with .iso extenstion, this is usually in the format of "rear-<hostname>.iso" which can be used as a bootable image file to recover system or even could be used to create a new system. This would fetch necessary backup files from the rear backup server.
By default these backup files would gets created under "/var/lib/rear/output/" and then would be moved over to nfs server.
After booting the iso file you could just select "automatic recovery" or select "recovery <hostname> option. I've made a small writeup on this in my blog: http://simplylinuxfaq.blogspot.in/2016/11/rear-linux-disaster-recovery-solution.html review it. This may help.
All the best!