Why In CW procedural signals, "K" is an invitation for any station to transmit, while adding "N" makes it restrictive: "KN" means go ahead, named station only. An operator sends it to keep the frequency with the station already in contact and discourage others from breaking in. It is commonly read as "over to you only" and is often sent as the prosign KN run together.
Watch out The idea that no US stations should call is not what it signals; that would be handled by other means, such as a DX station sending directional calls. Closing the station is signaled by "CL", and full break-in operation is announced with "BK" or QSK.
K = anyone may call; KN = No one else, just you.