A domain name under the top-level domain which abbreviated as TLD, such as google.com can be purchased on a domain name registrar or provider, if it is available and not yet registered by someone, nor reserved. A domain name registrar must be accredited by a generic top-level domain (abbreviated as gTLD) registry and/or a country code top-level domain (ccTLD for short) registry, such as Internet Corporation for Assigned Names and Numbers. (ICANN) With that registrar, you can register a domain name under TLD or second-level domain, abbreviated as SLD. For example in www.promos.tk web address, the name promos.tk is the domain name, and the .tk is the TLD, while the web address is the subdomain of the domain name. You have a full access in the domain name provider or in the web hosting service, to create any different names of the sub-domains, if you already own the domain name. And the example for the domain name under the SLD in www.yahoo.com.ph web address, the name yahoo.com.ph is the domain name, and the .com.ph is the SLD, while the web address is the subdomain of the domain name.
If a domain name registration service or domain name provider is not accredited by a 'generic and/or country code' top-level domain registr(y/ies), it cannot be legally called as a domain name registrar. And almost all of those providers are a free domain name registration service which was allowed their client or user to register a free subdomain under their second-level domain that's not a official second-level hierarchy for the TLD below it. For example in www.dungo.co.cc web address, the name dungo.co.cc is the free domain name, and the .co.cc paid domain is the SLD. The .co SLD is not a official second-level hierarchy for the .cc TLD. Somehow, there are some domain name provider which publicized that they are a free domain name registrar, even that they're not accredited by any TLD registry. Remember that a domain name registrar can be called as a domain name registration service, but a registration service that's not accredited by a TLD registry are cannot be legally called as a registrar. Some little of the domain name registration services that's not accredited are purchased a two-letter SLD under a ccTLD, and offer their paid domain to register a low-cost domain name under it.