A subdomain is a part of the main website, tech.china.com is part of the main website china.com. You lousy fuck.
If you have a large and confusing site thats hard to navigate, like china.com, then guess what you lousy fuck? You'll have to go with subdomains or subdirectories in order to improve user experience.
Google uses subdomains for distinct products such as news.google.com or maps.google.com, for example. Does that means news.google.com or maps.google.com are fake websites? No, you lousy fuck.