These notes in a reservation are generally provided by gate agents and other ground staff colleagues to give us a heads-up about your experience at the airport—such as, “3B missed their last flight because TSA held them up and he’s really angry” or “14A was mistreated by the last crew and the flight was delayed too, so please make sure they get the VIP experience.” Conversely, we often know about your pre-flight behavior—if you became verbally abusive, drank too much on your connecting flight (in which case we may choose not to serve you more alcohol), or were otherwise a nuisance to crew or your fellow passengers.
http://thepointsguy.com/2015/03/insider-series-what-flight-attendants-know-about-passengers/