From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 16-18:
As the war went on, the flow of ULTRA swelled like some great tidal wave, giving accurate and timely information on every aspect of the Japanese war effort, from the strategic to the domestic — not only Japanese war intentions, but individual ships’ machinery defects and junior officers’ promotions. The number of signals involved was enormous: the National Archives and Research Administration (NARA) in Washington DC has 290,908 decrypts of Japanese Navy signals on file, as well as many more thousands of naval attaché signal decrypts, intelligence summaries and daily digests.
ULTRA provided information broadly in four main categories. There was information of critical operational value, such as convoy sailings, warship movements, impending attacks, tactics and battle orders for on-going operations, which was directly applicable to current operations and provided in time for action to be taken on it. There was information of strategic value, such as intelligence of future operations, supplies, reserves, reinforcements and current strategy; on orders of battle, including the strengths, equipment and disposition of ships, aircraft and troops; and on Japanese intelligence, such as the results of Japanese spy activity, interrogations of Allied prisoners-of-war, captures of Allied documents, and the Japanese’ own traffic analysis, reconnaissance, and interceptions and decrypts of Allied signals.
Special arrangements were made for handling ULTRA. It was revealed only to certain Flag and Senior Officers and selected members of their staffs who had been ‘indoctrinated’ into the secret. When Arleigh (‘Thirty-one Knot’) Burke was Chief of Staff to Admiral Mitscher in the carrier Lexington in 1944, he was at first curious and finally angered by the mysterious behaviour of a junior naval reserve Lt (jg) who, alone of everyone on board, was allowed private and privileged access to the Admiral. Burke would see the two talking in low voices on the wing of the Admiral’s bridge, or sometimes withdrawing into the privacy of the Admiral’s sea cabin.
The officer was Charles Sims, Mitscher’s ULTRA intelligence officer, a Japanese language specialist trained in codebreaking, who gave Mitscher highly classified intelligence available through ULTRA. Burke himself was eventually admitted into the ULTRA secret, despite Sims’ protests, but not until special permission had been sought from CincPac.
The US Navy, unlike the Royal Navy, permitted the use of the word ULTRA in the text of signals. They regularly used some phrase such as ‘This is ULTRA’, but not habitually at the beginning of a signal, which would render it vulnerable to cryptanalytical attack, but always somewhere in the body of the text.
When information from an ULTRA source was passed on in another signal, that signal had to be paraphrased and so worded that, if captured or intercepted by the enemy, any reference to enemy intelligence could not be traced back to ULTRA. Any reference to the name of an enemy ship was to be avoided and any positions taken from an ULTRA signal had to be given in a different way.
ULTRA was so powerful a weapon that it often could not be used. Much of the information it provided could not be acted upon. Too many U-boats sunk at their remote fuelling rendezvous, for instance, would arouse enemy suspicions and imperil the ULTRA secret. Any operation undertaken as a result of ULTRA therefore had to have a ‘cover story’ — some corroboration from another source, such as naval or air reconnaissance, to account for the presence of Allied forces on the scene and at the time of the action. It was very easy, through an excess of zeal, especially in the Pacific, to make mistakes over this vital requirement.