App Units
The number of first-time app downloads made on the App Store using iOS 8 or tvOS 9, or later. App updates, downloads from the same Apple ID onto other devices, and redownloads to the same device are not counted. Family Sharing downloads are included for free apps, but not for paid apps.
Installations
The total number of times your app has been installed on devices with iOS 8 or tvOS 9, or later. Redownloads on the same device, downloads to multiple devices sharing the same Apple ID, and Family Sharing installations are included. Totals are based on app users who agree to share their data with you.
Sessions
The number of times the app has been used for at least two seconds. If the app is in the background and is later used again, that counts as another session. Totals are based on app users who agree to share their data with you.