A SubDomain is a domain name that is based on your current domain
name. An example of a SubDomain of somedomain.com would be
hellothere.somedomain.com. This feature is useful if you want to
divide up your site into different sections. The document root for
your subdomain will be the subdirectory that has the same name as
your subdomain prefix. For example:
http://testing.somedomain.com
is the same as
http://somedomain.com/testing
Therefore, you should upload to your subdomain accordingly.
Subdomains can be set up in your control panel. You will then need
to make a subdirectory in your public_html directory for each of
your subdomains.