The Backup role provides storage space for websites backups.
The backup role runs a separate instance of OpenSSH for data transfers on port 222.
Enhance backups are push backups - the unix user on the web application server is allowed to push to its counterpart on the backup server, however it does not have the ability to create or destroy backup snapshots (that is handled by the control panel server) so the web server could never remove backups from the backup server.
Website backups are incremental.... some information here.
We recommend installing the backup role on a completely separate server to your web servers and hosting it in a different data centre