CV_EVENT_MBUTTONDOWN what does this param mean?

edit

if you register a callback function to your window, CV_EVENT_MBUTTONDOWN is one of the events you can get here ( the 1st param in that function ) maybe you try to run the little test-program below, and just see, what happens ;)

void onmouse( int event, int x, int y, int d, void *p )
{
     cout << event << " " << x << " " << y << " " << d << endl;
     if ( event == CV_EVENT_MBUTTONDOWN ) cout << "middle!" << endl;
}
int main()
{
    VideoCapture cap(0);  // 1st cam
    Mat frame; 
    namedWindow("video", 1);
    cv::setMouseCallback( "video", onmouse );
    for(;;) {
        cap >> frame;
        if(frame.empty()) break;
        imshow("video", frame);
        if(waitKey(30) >= 0) break;
    }   
    return 0;
}