Pending notifications are notifications queued for Android or Electron devices which are currently offline. They might have uninstalled your app, turned off their device, or their device lost Internet connectivity.
Note: On iOS, only the last notification sent to the device is delivered when it reconnects. Other undelivered notifications are discarded. This is an Apple design decision.
If you attempt to send a notification to an Android or Electron device which is currently offline or has uninstalled your app, we queue it and label it a "pending notification". It's pending for the device to reconnect to our service.
Once a device reconnects, we immediately deliver all its pending notifications, in the order in which they were sent. Nothing needs to be done on your side in order to resend pending notifications, we do this automatically.
However, if a device has uninstalled your app and never reinstalls it, we will eventually delete the pending notification, when its time_to_live expires (by default, after 1 month, see the Send Notifications API reference).