Add Aliases for Subdomains
You are currently not able to Alias a subdomain through plesk. Instead you must create the subdomain as a primary if you wish to alias it. In many situations it would make sense from an organizational standpoint to keep the subdomain under a main domain while still adding aliases to it. I do not believe this is a technical restriction of the webservers so it would be helpful if plesk allowed you to add aliases to subdomains.