What are the differences between Child Domain Controller & Additional Domain Controller ?

I figure it would be best to use Microsoft's explanations for this.Child domainDomains subsequently created in a given Windows 2000 and Windows Server 2003 domain hierarchy become child domains of the root domain. For example, if msdn is a child domain of microsoft.com, the msdn domain becomes msdn.microsoft.com.http://support.microsoft.com/kb/310996Additional Domain Controllers If you already have one domain controller in a domain, you can add additional domain controllers to the domain to improve the availability and reliability of network services. Adding additional domain controllers can help provide fault tolerance, balance the load of existing domain controllers, and provide additional infrastructure support to sites.http://technet.microsoft.com/en-us/library/cc738032(WS.10).aspx

Does the child domain also automatic replicates the first DC(parent) just like the additional domain controller?Hengky