What is it for? It is in /var/www/clients/client1/web1/, the path is: /var/www/clients/client1/web1/backup As default instalation puts backups in /var/backup/ can't find any info about backup dir in client home.
When a client requests to download a backup in ispconfig, then the backup will be made available in the backup directory of his website.
Oh, got it... Have to click on Download button when logged in as a user, didn't test that option before