An Addon Domain allows you to host another Domain name within its own directory. It appears as a totally separate Web site from your main Hosting account. This allows the Addon Domain to have its own; Web pages, scripts, eMail addresses, etc. It is important to note, all Web space and other resources used by an Addon Domain will count as part of the main Hosting account.
Example:
40 MB of web space used by your main Hosting account
30 MB of web space used by an Addon Domain
—
70 MB total web space used
By default, the directory name will be the same as the Addon Domain. You can change that to name it whatever you choose. Best to keep it within 6 - 12 characters and always start with an Alpha-character (a-z). You can however, use Capitol letter (A-Z), numbers (0-9), dash (-), or an underscore (_) elsewhere in the rest of the name. Do not use special characters anywhere in a directory or file name!
The Addon Domain will use its own directory and have its own FTP login which you created upon setup. You always have access to it by using the login for your main account. By providing its own FTP login, this allows you to give someone access to the Addon Domain directory and only that directory, keeping the rest of your Hosting account safe from tampering. You can deny access at anytime, to everyone else but yourself, by simply changing the FTP password using:
cPanel > FTP > Manage Accounts.
By default, cPanel places the default value in the box as a suggestion. You can change that by inputting the path that you want.
If you want the Subdomain to be under addon-domain/sub-domain - then that is path you would input.
Make sure to use the correct directory name for the "Addon Domain" and create the name used for the "Subdomain" directory using the same rules above, for creating an Addon Domain.
When you create an Addon Domain using cPanel you should also remove or delete it using cPanel. This does not delete the directory created to store the files. That is a security precaution to prevent accidental removal of a directory, containing all the files related to an Addon Domain, in case you have not already downloaded them. After removal using cPanel has been done, you should download the entire Addon Domain directory and then delete it from your Hosting account, if no longer needed. This will free up the Web space previously used by the directory for the Addon Domain.