Delivery receipt can be used to see whether a message has been successfully delivered to all intended recipients by the Sendbird server. To mark messages as delivered when a group channel member successfully receives a push notification for the message from APNs, the markAsDelivered(remoteNotificationPayload:completionHandler:) method should be implemented in Notification Service Extension.
When a message is delivered to a group channel member who is online, it is automatically marked as delivered and other members who are online are also notified of the delivery receipt through the channelDidUpdateDeliveryStatus(_:) method of GroupChannelDelegate.