From Code Girls, by Liza Mundy (Hachette, 2017), Kindle pp. 244-246:
The language of the 2468 [transport code] messages was telegraphic in style. Short, straightforward, and no-nonsense, the messages consisted of sailing schedules, harbormaster reports, reports on the water levels of ports and transportation of cargo. Sailing schedules were the simplest. These included the transport number, the date, the time the maru [ship name] would be arriving or leaving, and its destination. Others concerned the movement of troops or equipment. A few dealt with transportation of the wounded or ashes of the dead. The marus out there in the Pacific Ocean carried everything: food, oil, supplies, human remains.
When a new message arrived, Dot looked for stereotypes, which were words that occurred frequently in the same place. “Maru” was a common one, but there were others as well, depending on the origin and the goods being transported. For example, one station transmitting from Singapore—the #3 Sen San Yusoo—sent a regular report on the shipping of oil to Hiroshima, Manila, and Tokyo. Stereotyped words might include ship names and numbers; the number of kiloliters of light oil, crude oil, heavy oil, aviation gasoline, or other gasoline aboard; how many trips each ship would take, and when. Another Singapore station transmitted to Hiroshima, Tokyo, and Moji a report of ships leaving for Palembang. Stereotypes might include the ship number or name, the date and hour of departure, the speed, the course, and the date and hour of scheduled arrival at the mouth of the Musi River.
Another transmitted a daily weather report with data including wind velocity and direction, temperature, and condition of the surface of the Andaman Sea, the South China Sea, the Yellow Sea, and other faraway bodies of water. Dot handled a lot of weather reports. Sitting at her table in Arlington, Virginia, Dot was amused at how many bits and pieces of information she knew about what the weather was like eight thousand miles away.
Another station transmitted a report on small boats available for supply services. Mentioned might be steel barges, wooden barges, special boats, small boats, twenty-metric-ton boats, plywood barges, and cargo submarines.
A station at Surabaya originated a report on the departure of ships escorted by a single Navy plane, including the names and types of ships (motor, sail, fishing), number of barges being towed, date of departure and destination, speed, scheduled date of arrival, route, and daily position of ship on consecutive days at given hours. A report from Shanghai about a supply ship might include a message spelling out that the probable route was “from Shanghai along the coast to the Yangtze River up the Yangtze River to WUU (Buko) down the river to Nanking and finally across the East China Sea to Moji.”
Here is how Dot did her work: Let’s say she knew that the code group for “arriving” was 6286 and she knew where this word was likely to appear. She would find that place in the message and look at the GAT ["group as transmitted"] before her. Books at Arlington Hall listed common code words as well as possible enciphered versions. She would look for a match, or she could do the math in her head and strip out the additive herself. Sometimes—when they were desperate—the code breakers would take the code groups and encipher them with every possible additive. A smattering of 2468 code groups included:
4333 hassoo—to send things [発送]
4362 jinin—personnel [人員]
4400 kaisi—beginning, commencing [開始]
4277 kookoo—navigate, to sail [航行]
4237 toochaku yotei—scheduled to arrive [到着予定]
4273 hatsu yotei—scheduled to leave [発予定]
There were vocabulary words associated with sailing schedules. According to training materials compiled at Arlington Hall, atesaki [宛先] was “destination” or “address”; chaku [着] was “arriving”; dai ichi [第一] was “first”; honjitsu [本日] was “today.” Maru [丸] was “commercial ship”; sempakutu [船舶通] was “ship” [-tu for 通行 tsuukou 'traffic, passage'?]; sempakutai [船舶隊] was “convoy unit”; teihaku [碇泊] was “anchoring”; yori [より] was “from”; yotei [予定] was “schedule.” Gunkan [軍艦] was “warship.” Chu [中] was “now [or 'in the middle of, i.e., underway'].” Hatsusen [発船] was “ship leaving.” Hi [日] was “day”; hongetsu [本月] was “this month”; senghu [sic; 船上 senjou?] was “onboard ship”; shuzensen [修繕船] was “ship being repaired”; tosai sen [搭載船] was “ship loading.”
Dot’s workday consisted of messages that, once deciphered, said things like “PALAU DENDAI/ 2/ 43/ T.B./ TRANSPORT/ 918/ (/878/)/ 20th/ 18/ JI/ CHAKU/ ATESAKI/ DAVAO/ SEMPAKUTAI/ 4/ CEBU/ E.T./”