In a trude code, symbols stand for ideas. A code attaches meaning to individual symbols. It is difficult to express oneself in code. A code has symbols only for the common words and messages that have been anticipated by the designer of the code.
Codes are awkward, often useless, in communicating the unexpected.
For that reason the important military, espionage, and diplomatic codes are ciphers. Labyrinths of Reason. An Elegant Enigma. Originally published by the NSA in 1978. Reprinted by Aegean Park Press in 1980.