userInfo

The user information dictionary stores any additional objects that objects receiving the notification might use.

For example, in AppKit, NSControl objects post the textDidChangeNotification whenever the field editor (an NSText object) changes text inside the NSControl. This notification provides the NSControl object as the notification's associated object. In order to provide access to the field editor, the NSControl object posting the notification adds the field editor to the notification's user information dictionary. Objects receiving the notification can access the field editor and the NSControl object posting the notification as follows:

func controlTextDidChange(_ notification: Notification) 
{
    if let fieldEditor = notification.userInfo?["NSFieldEditor"] as? NSText,
        let postingObject = notification.object as? NSControl
    {
        // work with the field editor and posting object
    }
}