Create a new domain tree only when you need to create a domain whose DNS namespace is not related to the other domains in the forest. This means that the name of the tree root domain (and all of its children) does not have to contain the full name of the parent domain. A forest can contain one or more domain trees. For information about how to create a new domain tree, see To create a new domain tree.
Before creating a new domain tree, consider creating another forest when you want a different DNS namespace. Multiple forests provide administrative autonomy, isolation of the schema and configuration directory partitions, separate security boundaries, and the flexibility to use an independent namespace design for each forest. For more information, see Creating a new forest.