From a purely technical perspective, everything is just a subdomain. www.example.com is a subdomain of example.com. Heck, even example.com is a subdomain of .com (which is a subdomain of the “root domain”, usually expressed as a single dot). And it works the other way around too. test.www.example.com is just another subdomain and there is nothing which will make it work any different from the standard www. subdomain.
Usually when we’re talking about subdomains, we’re talking about direct subdomains of the purchased domain name (e.g. blog.example.com), but in reality that’s just a way to make a very abstract concept more concrete.
Similarly, www.example.com is just another subdomain of example.com and no different from blog.example.com or shop.example.com. So why do we use it? It’s mostly about tradition. Back in the day, you’d always find a website on www.example.com, find a mail server on mail.example.com and so forth and there are plenty of people who still type www. before every domain. It’s because of this that web hosting systems give special attention to www. subdomains, not support incoming requests from www. to your main website could make you lose visitors.
But again, from a purely technical perspective, the whole www. subdomain is meaningless. So assuming you’ve got enough control over your systems (typically ruling out standard control panels), you could use any subdomain you wish. That’s how people can use www2.example.com or web.example.com and other uncommon variations like that.
@Admin well, this made it more clear, but still didn’t gave me an answer in which section in cPanel you can add a subdomain before www like test.www.
The free hosting control panel indeed does not support sub-sub domains. Premium hosting however does. You can simply enter test.www in the subdomain field and it will create the sub-subdomain for you. You can even create a regular subdomain first and then create a subdomain for that by selecting the subdomain in the domains dropdown.