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2. Procedural codes
3. Codes and ciphers
Presented for what it may be worth, with no claim for technical expertise, and not the slightest warranty!
Codes and Ciphers
There are (at least) three meanings for "code".
The first is International Morse Code, which is a modification of the original Morse telegraph system, in which a certain combination of 'dots' ('dits') and 'dashes' ('dahs') indicates a letter. It has the advantage of using only a very narrow segment of radio frequencies and is very reliable because all the transmitted power is concentrated in that narrow frequency range.
There are also codes in which letters, combinations of letters, words, or sentences indicate meanings.
Some are brevity codes. With those, the meanings of the letters, etc., are commonly known, and the code saves time (and sometimes money). For instance, SOS ( ... --- ... ) could be sent quickly by International Morse as a request for help. (The voice equivalent is the French "m'aidez", pronounced "Mayday".) QTH ( -- . - - .... ) means "What is your location?" (or "My location is _____"). There were large code-books to save money in undersea cable communication, so that TEKIY might mean "I have shipped your order by ocean freight" (but would be charged as one word).
Other codes are used for secrecy. Often they are also in large code-books. Great efforts have been made to get code-books used by enemy (or potentially enemy) countries, or to figure out the meanings of code terms. Others are made up for special purposes. One of the best-known examples of that was the word "Tora" to report success in the Japanese attack on Pearl Harbor. A much less well known example was the telephone message "The Italian navigator has landed.", reporting that Enrico Fermi had made a working nuclear pile at the University of Chicago.
While secrecy codes conceal meanings, a cipher (also often called a 'code') conceals individual letters or numbers by replacing them, one for one, with other characters. Typically, a cipher message requires a key that indicates how the replacement can be reversed, and great effort is devoted to discovering the key.
For more on codes and ciphers, a good start is: "The Codebreakers", David Kahn, Schribner, 1996.