App Settings- Push Notifications
The Apple Push Notification Service is a platform notification service created by Apple Inc. that enables third-party application developers to send notification data to applications installed on Apple devices. The notification information submitted can include badges, sounds, newsstand updates, or custom text alerts.
The Sandbox Key and passphrase for iOS is used to configure the notifications for the app in the development phase or a pre-production app.
The Production key and passphrase for iOS is used to configure the notifications for the app on a production app that is live for visitors.
While the FCM Server Key is used to pass your token from the FCM server to configuring push notifications on Android.