title: How is user_engagement event generated in firebase analytics?
body: <p>I have seen a param named <code>firebase_screen_class</code> (along with <code>engagement_time_msec</code>, <code>firebase_event_origin</code> and <code>firebase_screen_id</code>) in the <code>user_engagement</code> event in bigQuery, it contains the name of activity classes in my app. Please, can you explain these params and how is the <code>user_engagement</code> event generated?</p>
  <p>I also wonder whether this can be useful to get the quantity of impressions or screen views of the activities in my app.</p>