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Multiple Domains in Active Directory

Domains are units of replication. In addition, all of the domain controllers in a particular domain can receive changes and replicate those changes to all other domain controllers in the domain. Each domain in Active Directory is identified by a (DNS) Domain Name System domain name and requires one or more domain controllers. If your network requires more than one domain, you can easily create multiple domains.
One or more domains that share a common schema and global catalog are referred to as a forest. The first domain in a forest is referred to as the forest root domain. If multiple domains in the forest have contiguous DNS domain names, then the structure is referred to as a domain tree.
A single domain can span multiple physical locations or sites and can contain millions of objects. Site structure and domain structure are separate and flexible. Furthermore, a single domain can span multiple geographical sites, and a single site can include users and computers belonging to multiple domains.

Active Directory Field Guide
The domain is the principal unit of organization

Within a domain, objects can be organized into logical containers called organization units, or OUs

You can create more than one domain. Multiple domains can form a domain tree, and multiple trees can form a forest.

Mastering Active Directory
The root domain is always created first. It becomes the parent domain to child domains that are added directly below it.

Each domain in a tree is assigned a name using the hierarchical Domain Naming System, or DNS

As other domains are joined to the tree, the name of the child is added to the parent's name, reflecting their relationship.

Tree model of multiple domains can be extended to create a forest of trees for organizations that need to maintain separate organizational structures, such as a company that needs distinct public identities for its subsidiaries.