A subdomain refers to variations on the left of the domain, while the right part is the "root domain".
cameras is the subdomain while nikon.com is the root domain.
You can use subdomains for a Custom Domain, this is something you would set up on your side through your IT department.
The one exception to this is while technically www.nikon.com is a subdomain of nikon.com, there is a general convention on the web that it should be similar if not the same content as nikon.com. Accordingly, we support www.nikon.com if nikon.com is configured as the Custom Domain, but we do not allow www.nikon.com to be a Custom Domain if nikon.com isn't also a custom domain.
*Domains and/or subdomains can not be longer than 32 characters.*