When you add a new domain to your server, you are effectively creating space for a new website. Adding a domain creates a
web/example.com/ directory in which you can place your files.
An alias can be a sub-domain (e.g. 'www.example.com', 'demo.example.com') or a different domain name (e.g. 'example2.com'), pointing to an existing site on your server. Adding an alias does not create a new directory. Aliases are served from the same directory as their parent domain.