About 35 minutes into the animated film, Umi raised these five flags:
Shun, who was invited to Miki Hokuto's farewell party, understood the meaning of the signal flags: "H-O-K-U-T... Hokuto". What I don't understand is why didn't Umi raise the sixth O flag to complete the word? Is there a rule or taboo saying the sixth O flag can not be raised?
According to the Chinese Wikipedia, which cites page 18 of the Visual Guide, Umi didn't raise the sixth O flag because she only had one set of flags. Normally, substitutes can be used to repeat the same signal flag one or more times in case only one set of flags is carried on board:
The Repeater/Substitute flags allow any combination of five letters to be hoisted with only a single set of flags: hence, 2R means a duplicate of whatever was signified by the second flag.
However, in this case, they fall short of the required length (6) of the intended message (HOKUTO), and so were not used.