It was Morse Code (not just used by the military).
Tapping Code
tapping
mailSemaphore, message couriers, tapping, carrier pigeons, and sign language
Semaphore, message couriers, tapping, carrier pigeons, and sign language are all possible communication alternatives.
maritime communication is what was used for communication at sea for the military.
Tapping on a door or a window is commonly referred to as "knocking." This action is typically used to gain attention or signal one's presence before entering a space. In some contexts, it can also be a form of communication, such as in Morse code.
Military secrets.
point to point military communication
You must have heard about tapping of phone . Tapping data is same as tapping a phone , all the data entered into the database will be visible during tapping.
In-die tapping is the process of tapping stamped parts in a progressive die while in a punch press. In-die tapping eliminates the need for a secondary tapping operation of stamped parts.
yes it is, it is annoying
It means 'I have received and understood your communication'.