Monaro,
The subdomain isn't always set up like xxx.com/aaa
It really depends on the host. A lot of times it will be set up with a seperate ServerRoot. A lot of paysites will set up the "members" subdomain on a seperate server entirely from theire public or tours. Then if any hack attepmts happen on the main site, no content is compromised, and all the current paying members are still able to log in and get their porn.
If you just want a subdomain as a way of accessing a section of the site, you can do it with a redirect in an .htaccess file, and point it to the directory, so you don't even have to involve your host.