The unique string assigned to the category.
- iOS 10.0+
- macOS 10.14+
- watchOS 3.0+
Use this string to differentiate the different types of notifications that your app can send. To assign a category to a local notification, assign this string to the
category property of the content object. To assign a category to a remote notification, use the string as the value of the
category key in the
aps dictionary of the notification payload.