You want to say something about another person's status or post, but don't want your comment to pop up in their Notifications. So, you use sub-booking: instead of adding your comment to their discussion, you just post it to your own status. Your friends all see the comment, but the person you're commenting about doesn't get an alert and, if you filter your friend groups, doesn't necessarily see your comment at all.
by 155fathoms June 24, 2013