OneSignal only counts a session after the user has left the app for over 30 seconds. If you close the app or website and return to it within 30 seconds, it will not be a new session. For instance, Apple's analytics tracks the session as 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. Learn more about sessions by checking out our analytics documentation.