Nomenclature
The cipher is named after the six possible letters used in the ciphertext: A, D, F, G, V and X. These letters were chosen deliberately because they sound very different from each other when transmitted via Morse code. The intention was to reduce the possibility of operator error.
Nebel designed the cipher to provide an army on the move with encryption more convenient than trench codes but still secure. In fact, the Germans believed the ADFGVX cipher was unbreakable.
Read more about this topic: ADFGVX Cipher