Why would you create a new forest?

A new Active Directory forest is basically a completely new Active Directory environment. When you create it, it does not have a relationship with an existing Active Directory environment, unless you choose to create Active Directory trusts afterward.

Since the new Active Directory forest is separate, a boundary is created for the following reasons:

• Schema and configuration partitions: The schema and configuration partitions hold information on the way that objects can be created, what attributes are required for these objects, what attributes are optional for these objects, and the domains within the forest. Since many applications require Active Directory schema extensions, introducing a legacy or cutting-edge application might result in schema conflicts. In these types of scenarios, creating an additional Active Directory forest is the best way forward. An alternative might be to add an Active Directory Lightweight Directory Services (AD-LDS) instance to the environment.

• Global catalog replication: Domain controllers with the additional global catalog role hold partial information on the most requested attributes for objects in Active Directory. With multiple global catalogs, the information is replicated throughout the forest. To shield this information, an additional Active Directory forest can be created.
• Forest DNS zones replication: To overcome the default boundary for Active Directory-integrated DNS zones, the Forest DNS zone replication scope, an additional Active Directory forest can be created.

When requirements apply in terms of schema or replication, creating an Active Directory forest is the right choice. One thing that might be good here is to state that the forest is a security boundary as well as an administrative boundary.

Additionally, since the forest is a separate environment, by default, it can also be separated afterward. In acquisition and divestiture scenarios that can be overseen for the life cycle of Active Directory, an Active Directory forest is also the right choice.