Semaphore is an evolution of the optical telegraph, which was used to send messages across vast distances.
The name "semaphore" also can refer to signaling with a light or other means. This encoder uses the flag method and does a very basic translation of characters. When signaling, you should signal Error or Attention. During sending, the receiver will signal "C" with each word properly received, or "E" if there was a problem. When problems are detected or when cancel is received, then the entire previous word is repeated. Double letters should get a very brief space inserted. Switch to numbers by using a "#" in front. Switch back to letters with "J".
These images show the sender, and need to be reversed when you are the transmitter. So, the letter "B" is signaled by raising your right hand halfway up. Also, the letters A-I plus K are used for 1-9 and 0 (the letter J was added later).
The font is a modified version of Semaphore Pramuka. This version uses "!" for error/attention, a space for the rest symbol and to split double letters, tilde or slashes for cancel.