While the operation system installed for SBS servers is Windows Server 2008, it has some limitations to ensure that it is both properly used and to protect non-technical users from implementing what they may not understand to be overly complicated designs.
The Windows SBS 2008 network may contain only one Active Directory domain and that domain cannot have trusts established with other domains. Subdomains are similarly prohibited. For an environment as small as the 75 user limitation in SBS, this is the only reasonable option anyway. The SBS 2008 server must also act as the domain controller for the AD forest, even if another Windows Server is brought into the network for other purposes.
One, and only one additional SBS 2008 server can be in the Windows SBS 2008 domain, and the “second server” license that comes in the Small Business Server Premium Edition cannot be used as a stand alone server. It must be joined into the primary Windows SBS domain.
Finally, the included applications like Exchange 2007 and SQL Server 2008 Small Business Edition are only licensed to be installed on the SBS servers regardless of whether any additional separately purchased Windows Server 2008 machines are brought onto the network.