Active devices are defined based on your app's platform:
iOS: Any user that has launched your app at least once on their device during a monthly billing period.
Web Push: Any user that has visited your website at least once on their device during a monthly billing period.
Android: Any user that has installed or kept your app installed on their device during any point of a monthly billing period.
Users must be registered for notifications (by calling Pushy.register()) before we start to bill you for them.
Related: How we identify devices uniquely across app reinstalls