Sometimes it may be reasonable to use subdomains of other domain names that you have registered, but I would think that sometimes it would not be appropriate, s

Sometimes it may be reasonable to use subdomains of other domain names that you have registered, but I would think that sometimes it would not be appropriate, such as if you are not using it with internet at all and therefore should not need to register a domain name, or for other reasons; if it is not necessary to use internet domain names then you would likely want to avoid it (or, at least, I would).