The localized subtitle, containing a secondary description of the reason for the alert.
- iOS 10.0+
- macOS 10.14+
- watchOS 3.0+
Use this property to specify additional context about the purpose of the notification. Subtitles offer additional context in cases where the title alone is not clear. Subtitles are not displayed in all cases. If your app is not authorized to display alert-based notifications, this property is ignored.