web root is the root directory that the web site is in. If you were creating a cert for the OMV web interface, it would be /var/www/openmediavault/
domain is the web site's public domain that points to your server. Not sure how to describe that any differently.
Ok, I got it
If I want to generate SSL cert for "a service" different than omv webGui?
I mean I expose some services like ipcam manger webGui and so on, how can I generate SSL cert for this services?