Chosen for how easy it is to type on virtually all cellphone keyboards (its the first choice on both T9 and iTap, and K is directly above M on qwerty keypads), Km does not stand for anything although legend states that it may stand for "Keep this Message private".
After a Km message is read, it must be deleted from the receiver's phone -- allowing a Km message to be read by someone not designated for the message is a severe violation of Man Law, as decided by the Man Council Mach II.
Km is traditionally placed in the first line of a message, followed by a few spaces or line breaks so the reader may comprehend the meaning.
Km applies to only one message, and it pertains to the entirety of the message -- partial Km's and Km's for more than one message are not allowed. Km must be declared in each message that is to remain private.