DNS zones and how do they count
A DNS zone is the portion of a domain, hosted on a specific name server. Each domain's zone contains the DNS records - the special DNS record type Start of Authority (SOA) and all the other records for the resources described within the zone.
DNS zones for sub-domains
A sub-domain could also have its own DNS zone. Sub-domain DNS zones bring all domain DNS features to the sub-domain level.
How do zones count?
Here at ClouDNS, you can create a Master DNS zone, a Slave zone, a Free zone, a Reverse zone and a Parked zone. Each one of these choices counts as a separate zone, bulk zones and zones for sub-domains also count as separate zones.
NOTE: If you have a registered domain and do not have a zone for that domain, the domain itself does not count as a separate zone.