This function been around since Window Server 2003 NPS, however I'm struggling to find this in the Clearpass Policy Manager.
The Clearpass Policy Manager is the Radius server.
RADIUS Access-Request messages are processed or forwarded by NPS only if the settings of the incoming message match at least one of the connection request policies configured on the NPS server.
If the policy settings match and the policy requires that the NPS server process the message, NPS acts as a RADIUS server, authenticating and authorizing the connection request.
If the policy settings match and the policy requires that the NPS server forwards the message, NPS acts as a RADIUS proxy and forwards the connection request to a remote RADIUS server for processing.
- explanation taken from http://msdn.microsoft.com/en-us/library/cc753603.aspx