Sub-domains are not separate unique domains, for example, en.wikipedia.org is a sub-domain of wikipedia.org domain name. They are saved under public_html as a folder when it is created via cpanel. Here instead of www in the web address we add the sub-domains rather than creating a new domain name.Thus Sub-domains are simply a directory in public_html. We can use it either:
subdomain.yourdomainname.com
(or)
yourdomainname/subdomain.com