Active Directory Administration Cookbook

Why would you have a new domain?

A new Active Directory domain—as either a subdomain of an existing domain, or a new domain tree in an existing forest—provides a boundary.

The boundary of domains in Active Directory relates to the following:

• DNS name: An additional domain tree offers the possibility to add a DNS domain name to the organization to, for instance, correctly label a new business venture. An alternative might be to add an additional UPN suffix.
• Domain DNS zones replication: Throughout an Active Directory forest, all domain controllers replicate to exchange information on objects, schema, and configuration. Between domains, a distinction can be made to limit the replication of information on Active Directory-integrated DNS zones. ...