FQDN stands for Full Qualified Domain Name.
Subdomains are NOT Full Qualified, onty top tld domain names are welcome to that, so Apache test cannot tell you NEVER the oppossite.
Example: server1.xxx.com
xxx.com is FQDN, a top level domain (tld).
server1.xxx.com is 2ēnd level domain, usually known as subdomain.
cdn.server.xxx.com is 3ērd level domain, you can call it also a subdomain, but deeper.
Note: if you buy somedomain.com.es MAY be threated as NOT FQDN on many tests even if it's really a TLD, because com.es is recognized as subdomain if you use a string comparison operation. These tipe or pseudo top tld has to be bypassed on those each test you submit it.
As example, i got rejected on many listings to try use .com.es. (NO MORE)
Think about internet and tech are not fine tuned.