It's not that much documentation because the use is pretty simple.
Let me explain you quickly, say your domain name is howard.com and mailwizz is installed there.
Now, howard.com is hosted on a dedicated ip, say 188.8.131.52 so that when you access http://184.108.40.206 you get the exact same thing as you get when you access howard.com, that is, the mailwizz application.
Next, let's say you want to create a tracking.howard.com so that all your email links point there instead of howard.com directly.
You then create a DNS record of type CNAME for tracking.howard.com to point to howard.com, so now when you access http://tracking.howard.com you get back, like in the above 2 cases, the mailwizz application.
You could also add a ServerAlias tracking.howard.com to your apache vhost for howard.com in case when you access tracking.howard.com you don't see mailwizz and you see the default apache website on the server.
Now, you add tracking.howard.com as a tracking domain in mailwizz, you verify it and then assign that domain to a delivery server. Doing like so, all emails sent through that delivery server will use tracking.howard.com as the domain for each link.