An addon domain is specific to particular types of hosting accounts, for example, cpanel. Its not a universal function to all web hosts.
A Cpanel 'addon' domain is already IN a subfolder of the main domain. You cannot create subdomains on
addon domains. You will need to work with real domains and real subdomains instead (get a 'reseller' type account). The only good thing about addon domains is they can save you money hosting, at the expense of normal domain options, and are more a choice for pointing domains to one page than a choice for building a business on.
Here are various bits of info I have gathered over the years....
Add-on domains also are all managed within one control panel and cannot be used with pre-installed CGI-Scripts or the Fantastico panel.
It is important to understand that your add-on domain will rest within a folder in your main directory. For instance, these domains point to the following folders:
What is an addon domain?
An addon domain is a fully functional domain that will work out of a folder in your main sites ftp.
Addon domain is a completely separate domain for your visitors, but for you it is inside your main domain and has no control panel of its own.
An add-on domain is a new domain name that points to a subdirectory within your existing domain hosting account, where the website for the new domain will reside.
The advantage of add-on domains is that the browser's address bar will show
(there will be no reference to the original domain), so the process will be totally transparent to your users. If your
users navigates to another page, their browser will accordingly show
[add-on-domain.com...] just like it should.
Can I create email accounts, databases, ftp accounts and subdomains on addon domains?
A - No. You can't create separate databases, ftp accounts and subdomains on addon domains because "addon domains" is simply a pointer to sub directory of
main website "public_html" folder.