Domain
A Domain provides a way to "partition" the reporting database into separate, private realms. It is used primarily to implement security controls for multi-tenancy systems.
See Multiple domains (for managed services partners only).
A Domain is a unique entity associated with the top level of your host group hierarchy. The domain name is supplied during the installation process and the Portal assigns it to the root folder.
The domain is used by the Data Collector for:
Authentication: The primary server record must exist in the host group hierarchy for the domain. (Veritas NetBackup only)
Host Searches: The Data Collector searches the domain's host group hierarchy checking for hosts associated with the backup data it is gathering. If no host is found, a new host is added to the root-level host group folder for the domain.
Enterprise environments typically have only one domain. When you add (or delete) host groups or attributes, you do so globally for your domain and all the host groups in that domain. Unless you are a Managed Services Provider (MSP), you should not have to specify a domain when you add or delete host groups or attributes through the Portal.