Cloud (or bulk) domains is an internal feature of ClouDNS that allows you to manage the records of your multiple domains. If you have many domains with the same records, you can add them to the cloud of yourdomain.com. After that, if you make any changes to the records of yourdomain.com, all the changes of it will be reflected to your cloud domains. For example: If you have many domains, pointed to the same IP address, you can use Cloud domains.
If you don't know how to create Master DNS zone, you can check it here.
How to use it?
You are ready! The domains are added to your Cloud and they have the same records as these in your Master DNS Zone.
How to add Cloud Domains in ClouDNS - Step by Step: