Domain Functional Level refers to a set of attribute values that help applications to determine the DS-BEHAVIOR-VERSION, also called as functional level of the domain.
To Validate the Domain functional Level: In the MMC, click Active Directory Domains and Trusts, right-click Domain, then select Raise Domain Functional Level.
A message appears stating the Domain functional level Windows Server 2016 for the updated server.
To Validate the Forest Functional Level: In the MMC, right-click Active Directory Domains and Trusts, right-click Domain, then select Raise Forest Functional Level.
A message appears stating the Forest functional level Windows Server 2016 for the updated server.
NOTE:Windows Server 2016 functional level has DS-BEHAVIOR-VERSION value as 7. With AD 2019 schema, there is no change in Functional level from past release AD 2016.