How do addon domains work?
Even though addon domains do not have their own control panel, in general they have the same functionality as the primary domain.
Technically addon domains point to a subdomain/subdirectory on your primary domain. However they function just like any other domain from the visitor's/browser's point of view. If you have an addon domain "addon.com" and your visitor types "www.addon.com" into the browser, the browser's address bar will show "http://www.addon.com" and not "http://www.maindomain.com/addon". If your visitor navigates to another page, the browser will accordingly show "http://www.addon.com/anotherpage.html", just like it should.
You can create separate email accounts based on your addon domains as well.
You can manage your addon domains in your primary domain's control panel.