Since cPanel has started offering limited number of cPanel and changed its pricing based on number of cPanel accounts, addon domain has become more popular.
Addon domain works as same as an independent website and only difference is that it would be managed under main cPanel only. It would share all resources for the allowed cPanel resources for space, transfer, number of email id and everything.
How to create an Addon Domain in cPanel
1 - Login to your whm and ensure that your cPanel package is allowing you to create addon domain, if no, then either you will have to create a package with addon domain and assign that cPanel to the package or you need to ask your hosting provider for the same.
2 - Once it is done, login to your cPanel and search for addon domain. You will see the section to create addon domain.
3 - Point your domain to your server and create addon domain in cPanel. You can either choose to create a new ftp or you can use same master cPanel login to upload files.
4 - Once created, it will create a domain name folder in your main cPanel too upload your all files and folders in the same.
5 - You can use .htaccess file to redirect your urls as needed. And can also create your email id and everything that is needed via same cPanel.
6 - You must keep a limited number of addon domain or standard sites to your single cpanel to avoid memory and cpu usage issues.
How to Remove an Addon Domain in cPanel
1 - Login to your cPanel and go to addon domain section.
2 - Carefully select your addon domain you want to remove.
3 - Click on the remove link to delete that Addon domain.
4 - If you want to delete database and database user, associated with the removed addon domain, you will have to go to mysql database section and remove DB and its user, if needed.