What type of trust is new to Windows Server 2008 and is only available when the forest functionality is set to Windows Server 2008?

The new type of trust introduced in Windows Server 2008 is the "R2 (or Forest) Trust." This trust allows for a direct relationship between two forest root domains, enabling resource sharing and authentication across forests. It is only available when the forest functional level is set to Windows Server 2008 or higher, allowing for enhanced collaboration between separate Active Directory forests.