What does "main screen" actually refer to, in battery usage?
From https://www.apple.com/batteries/maximizing-performance/#ios:


  Home &amp; Lock Screen. This indicates that the Home screen or Lock screen
  was displayed on your device. For example, the display was awakened by
  pressing the Home button or by a notification.
  
  
  If an app frequently wakes your display with notifications, you can turn off push notifications for the app in Settings > Notifications.
  Tap App and set Allow Notifications to Off.