When you subscribe a Realtime channel with push notifications (using the subscribeWithNotifications method), the Realtime servers will automatically make a subscription for your device in APNS (iOS) or FCM (Android). 

As soon as that subscription is made, any message sent to the subscribed channel will be sent through the Realtime connection (if the user is connected) and also to the APNS/FCM platforms. 

This means that if the subscribed device is not connected to Realtime at the moment the message was sent, APNS or GCM will deliver a push notification containing the sent message to the device.

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