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The second parameter can be used to report some progress to the antuser. For example you can report some messages, which might be interessting. You also can report a progress, which will be display as a progressbar. The IProgressReporter can also be used to Log your progress. Your logs will be present in the antony logs.
In the current Version (2.21.130) of antony the messages within the IProgressReporter aren’t displayed to the antuser. But in further versions of antony these informations might be displayed. So don’t write some crap in it 
Conclusion
As you saw it is right simple to provide a custom button within the chat. You can use this method to create some easy but powerfull integrations. Perhaps you want to search for an ordner within an ERP system to put some detail informations to the chat. Perhaps you want to create a message with ticket details to be dispartcher.