channel2 is a Webservice interface that offers access to the incident data stored at a Naggregator Instance.
If you build Naggregator, you will find the WSDL in the project/ticket/webservice/channel2-VERSION.jar at META-INF/wsdl/Channel2Service.wsdl.
A documentation of the interface itself can be found in the JavaDocs (the interface itself is here).
The Samples & Snippets package contains a sample called Incident Roundtrip that illustrates the use of channel1 and channel2.