On Sept. 24, 1941, Tokyo sent a message to their people in Hawaii asking that future reports of ships in harbor be broken down by areas. The message was christened "bomb plot", a nickname which conjures thoughts of conspiracy and plotting. However, we cannot go solely on a name assigned to the message by US investigators.
See the message.
While the "bomb plot" message does lay out the harbor into distinct areas there is no evidence that is was to be used for bombing. To be useful for such there would need to be up-to-date information available to the pilots, from direct observations taken literally hours or minutes before the attack. No such information was sent from Hawaii in the hours before the attack.
The last communications from Hawaii to Tokyo doesn't even use the "bomb plot":
"On the evening of the 5th, among the battleships which entered port were (garble) and one submarine tender. The following ships were observed at anchor on the 6th: 9 battleships, 3 light cruisers, 3 submarine tenders, 17 destroyers and in addition there were 4 light cruisers, 2 destroyers lying at docks (the heavy cruisers and airplane carriers have all left) * * *."
We would also have to assume that the pilots would need this information to determine where their targets were. As they would have an aerial view of the harbor on the approach it doubtful they would have needed second hand reports of where to look for battleships or carriers. Questions about the value of the "bomb plot" information should also be considered in light of the fact that Utah, a ship of no combat value, was attacked despite the pilots having been advised not to waste ordnance on her.
As the Joint Committee Report puts it:
"Failure to use the plan for indicating the location of ships within the harbor at the only time when it could have materially assisted the attacking force in locating ships as targets for bombing, that is on December 5 and 6 immediately before the attack, raises a serious question as to whether the berthing plan was in reality a bomb plot at all. " (p. 188)