If say you have 5 domains in Site A and one domain in site B then only AD information about that one domain would replicate to site B. If say you had a GC in site B then the entire forest would ...

Step 2 : then run the following command from the Schema Master role holding DC - dc01.contosoholding.com (forest root domain) Setup.exe /IAcceptExchangeServerLicenseTerms_DiagnosticDataOFF /PrepareAD ...

No minimum AD domain or forest functional level (DFL/FFL) is required. The software doesn’t create or require accounts in the AD domains that it protects.