Webhosting Subdomains
Sub domains are handled specially on the server. Each sub domain on the web appears as a sub directory in the filesystem space. The web site will have a root directory structure that you can access by ftp. If you have a password for the main domain (e.g. example.com) but logged in by ftp to a sub domain (e.g. www.example.com) then you find yourself in a www sub directory. Each directory in the file system is treated the same as a sub domain. So if you made a sub directory 'test' with a directory in it called 'thisisa' and then set up a suitable DNS record for thisisa.test.example.com (a CNAME to www-server.uk.) then that would refer to the '/test/thisisa' directory. Mostly people only use www as a sub domain and that is the directory in which they put their site. Obviously sub directories also work as normal, i.e. the previous example would work when accessed as test.example.com/thisisa.
Remember that 'www.' is a sub domain already, so other sub domains probably shouldn't be under www.