In a resource forest topology, Office Communications Server is deployed in one forest, a resource forest that hosts Office Communications Server servers but does not host any logon enabled user accounts. 

Outside of the resource forest, user forests host enabled user accounts but no Office Communications Server servers. Within the resource forest, a corresponding disabled user account exits for each user account in the user forests.

Part 2 of this document explains how to configure Office Communications Server 2007 R2 to support a resource forest topology.