I understand subdomains, ie subdomain.maindomain.com.
However, I'm not sure what an aliase is. I assume, that it allows me to access maindomain.com using aliasdomain.com instead. Thus allowing two domain names to share the same code etc.
If the above is true, then, judging by what you said, I guess "Addon" domiains are not supported.
To explain my understanding;
maindomain.com accesses the index.html file in the public_html folder.
subdomain.maindomain.com accesses the index.html file in the public_html/subdomain folder.
aliasedomain.com accesses the index.html file in the public_html folder.
addondomain.com accesses the index.html file in the public_html/addondomain folder.
This has implications for the coding of my site, as I am making use of the Addon Domain functionality allowed by my hosting company already. (I am preparing migration to a VPS server.)