The primary text is the 戦史叢書 (senshi-sôsho) which details all the strategies and plans for the Japanese army and navy throughout WW2. This text has 68 chapters for the army, 33 for the navy and a single chapter of list of ‘combined operations’. In this he MI operation for example is referred to the ‘central Pacific naval operations’ and so on. So the alphabetical abbreviations are immediately suspect and probably applied post-war to organise documents. Added to this - the Japanese never felt the objectives in war - Midway, Burma, Singapore and so on to be a mystery. They thought it was obvious where the battles would be - and the more sensitive information (when, how, etc.) would remain secret. Furthermore, operations names served a logistics and moral function (to requisition and inform junior officers) and said nothing about the specifics of the operation as they had plans for everything (invasion of Australia plan) and the plan alone meant nothing. Attack codes and operational codes were actual codes (usually referencing the Russo-Japanese war or Japanese mythology).

Secondly, the Japanese never knew their codes were broken, always believed Japanese was too difficult for foreigners to comprehend and did not fear espionage from the ranks (like the allies did). So this also often contributed to a general lack of imagination when it came to broad strategic operation names.