In case if you want to host two or more sites with two or more domains on the same hosting account and you may want to register and market another domain name but don't want to pay for another hosting account then the best solution is to create an add-on domain. An add-on domain is a new domain that is added on top of your main domain.

 An add-on domain is a domain name that that points to a subdirectory within your account. It is a fully functional domain that will work out of a folder in your main sites ftp.

 Add-on domains are basically like having separate accounts all within one control panel. Each add-on domain will have its own web pages, email accounts, mysql database, and ftp access. However, all domains will be controlled through you current CPanel. It shares all the same resources (disk space, bandwidth...) with your main site. It has its own FTP manager, site stats, cgi-bin which is created automatically

 With this feature in Cpanel you can create a separate domain next to your main domain on your account. Every add-on domain also creates a sub-domain. An Add-on is basically a sub-domain that can also be accessed by the full domain. So From a web user perspective, an add-on domain functions just like any other domain for example:

 If you would add a domain named abc.com and your main domain is xyz.com, Cpanel creates a sub-domain called abc.xyz.com. The content which goes in the /public_html/abc/ directory will be visible via your browser under: abc.xyz.com AND abc.com

 Add-on domains must be registered domain names that you own and must be configured to point to our name servers in order to work. It will look completely independent to the outside world with no means for anyone to know that this is running on an account under another site.

 
Thus, the Add-on domain feature is the best solution to optimize the Web Hosting Space.