This is used by a controller to notify other controllers about a modification in the state of groups and meters. It utilizes the OFPT_REQUESTFORWARD message. It should be noted that this is enabled on a per-controller basis.
The heart of OpenFlow specification is the set of C structures used for OpenFlow protocol messages. Those of you interested in this can find these data structures and their detailed explanation available at http://archive.openflow.org/documents/openflow-spec-v1.0.0.pdf.