You shouldn't accept 'Add-on Domains' as Multiple Domain Hosting. It isn't true Multiple Domain Hosting. However, 'Add-on Domains' is the tool that will allow you to point domains into sub-directories of an account on a cPanel server though.
The difference between addon domain and true multi domain hosting (if you have Cpanel) is that the addon domain uses from the same resources as your main domain (data transfer/databases etc.), does not get its own email accounts or control panel. You do get subdomain logs but you have to download them and analyze them on your computer.