...and in some cases you don't even need that. If I know your booking reference number and your last name (say, by hunting through the garbage bins in the office after you have left for the night, looking for the printout of your flight confirmation before you moved yourself into a preferred set) I can go and move you out of 18A into 60E thereby opening up 18A for me (OK I know you never fly economy jaysona but you get the general idea)