Enabling Advanced Features for AD DS
Active Directory Domain Services (AD DS) makes it possible for you to introduce advanced features into your environment by raising the domain or forest functional levels. To use advanced AD DS features, you must identify the operating systems that are running on the domain controllers in your environment.
You must also determine the best functional level for your organization based on your existing infrastructure and then raise the domain or forest functional level as appropriate. You can raise the functional level when all domain controllers in the domain or forest are running an appropriate version of Windows. Although raising the functional level makes it possible for you to enable new features, it also limits the versions of Windows operating systems that you can run on domain controllers in your environment.