Full enumeration is useful for security research. For blocking purposes it can sometimes be helpful to enumerate it. And on your question: it is most often the

Full enumeration is useful for security research. For blocking purposes it can sometimes be helpful to enumerate it. And on your question: it is most often the case that a subdomain is owned by the same entity, but it is not a given. The most common thing is a CNAME to some site of a vendor. But you can go all the way to full subdomain delegation (this is the same as going from com. to facebook.com.).