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The functional level of a domain or forest determines the advanced features that are accessible within that domain or forest. By increasing the functional level, new features become available that may provide additional security options.
The Domain Functional Level (DFL) should be compliant with the oldest Windows Server version of the domain.
Forest and Domain Functional Levels
Understanding Active Directory Domain Services (AD DS) Functional Level
Name: Domains with an Outdated Functional Level
Codename: C-DOMAIN-FUNCTIONAL-LEVEL
Severity: Medium