The website I’m looking to find this for is rubiconins.com. Would the FQDN be “Cloudflare.rubiconins.com.”?
Thanks
No. The answer is:
EDIT. This is actually incorrect. Correction below.
Technically FQDNs should include the hostname such as mailserver.example.com
hostname = mailserver
domain = example
top-level domain = .com
However, often folks will say “FQDN” in reference to just domain + tld
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I think technically the domain can represent the FQDN if that also identifies the machine/host in question (example.com).
Appending to your previous answer @anon13938084 technically it should also include:
root = .
So a FQDN really is mailserver.example.com. but no one really does that (including the trailing .) outside of perhaps in a BIND file because the root is implicit. And for completeness I guess technically there can also be n instances of:
subdomain = $foo
So server.us.example.com or server.us.corp.example.com which would be two different FQDNs. Although outside of Active Directory instances of multiple subdomains are pretty rare. 
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By RFC 1983 FQDN should include subdomain. My mistake then. I was going to say that both of them are technically FQDNs but didn’t check RFC and finally made wrong assumption.
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@kevaleb we’re just arguing semantics now for fun, the previous answer was correct. 
I would argue the distinction in in that RFC is between a host name and a FQDN is an example not a rule. And that the example is intended to demonstrate the difference between a FQDN and a PQDN. When there is an @ record at the root that specifies a host I think that technically it is fully qualified and that the host ‘@’ is implicit in the FQDN example.com in the same way the trailing root ‘.’ is also implicit.
If you don’t have an ‘@’ A record (which is common in Active Directory for example not to have a host which represents us.corp.example.com) then that doesn’t represent a FQDN, but instead server.us.corp.example.com would be a FQDN. 
Thanks for your help guys.