SmartFile allows branded sites to use their own sub domain. In many cases users are primarily using SmartFile for FTP so it only makes sense they would chose ftp.mydomain.com as their sub domain. However this is the only sub domain not allowed. The reason is because of how browsers handle the protocol.
Browsers assume the http protocol. This allows you to type in mydomain.com or www.mydomain.com in your browser and it will load your website. The problem is that browsers are also capable of FTP. This means when you type in ftp.mydomain.com it doesn’t force the http:// but now connects you to ftp://ftp.mydomain.com. You would then be prompted for a username and password and would be logged into a Windows Explorer type of FTP. This would still allow you to upload/download files but
without all of the features that SmartFile provides. Now some will argue that you can still use the SmartFile application by typing in
http://ftp.mydomain.com , which is true. But are you willing to take the chance that your customers will remember to type http:// every time? By choosing any other sub domain you don’t have to.