/DomainLevel:{0|2|3|4} The domain functional level cannot be lower than the forest functional level. Default will be automatically ...

/DomainLevel:{0|2|3|4}
The domain functional level cannot be lower than the forest functional level. Default will be automatically computed and set to the existing forest functional level or the value set for /ForestLevel
Specifies the domain functional level when creating a new domain. A value of 0 specifies Windows 2000. A value of 2 specifies Windows Server 2003. A value of 3 specifies Windows Server 2008. A value of 4 specifies Windows Server 2008 R2.