Difference between Addon domain & Parked domain
Addon domains can be added to your hosting account as an additional feature. They allow you to have more than one independent website sharing the resources of your hosting account. You can host multiple domains or multiple websites on a single account which are basically set up as subdomains on the main domain but the domains you add as an addon domain gets parked on top of the subdomain. You can see the website of the subdomain when you type your addon domain in the browser by the URL never changes as it continues with the add-on domain in the browser.
You can have multiple domain names pointing to your site. This is a great way to increase your online visibility. Parked domain on main domain shows the website of the main domain. Similarly parked domain on the subdomain would show the website of the subdomain. A domain that is parked on the subdomain is called an add-on domain. Apache configuration needs to know what directory to call when someone opens a particular website in the browser and in the case of addon domains, it will load the subdomain. Your addon domain is added as an alias for that subdomain. From a user perspective, an addon domain functions just like any other domain. You can register and set up an addon domain under your existing web hosting account.