Please HC7 Upgrade this
When you create a subdomain the HC Add a NS Record on DOMAINNAME.com and it makes subdomain.DOMAINNAME.com DNS ZONE
HC do all this job and can just add a A record on DOMAINNAME.com with the subdomain name like
[SUBDOMAIN] A [IP]
or
SUBDOMAIN.DOMAINNAME.net. IN A [IP]
this is just a comment