/SiteName:"site_name" The default value depends on the type of installation. For a new forest, the default is Default-First-Site-Name. ...

/SiteName:"site_name"
The default value depends on the type of installation. For a new forest, the default is Default-First-Site-Name. For all other installations, the default is the site that is associated with the subnet that includes the IP address of this server. If no such site exists, the default is the site of the replication source domain controller.
Specifies the name of an existing site where you can place the new domain controller.