The flag signal is a message made up of flags raised in a specific order. It allows ships to visually exchange messages without any special equipment. Each country has its own flag alphabet. Three different flag signals can be seen in From Up on Poppy Hill film.
Umi's flag signal[]
Umi's flag signal
Every morning Umi Matsuzaki raises two flags which messages "I wish you a safe journey" to all passing by ships in universal flag language. She was taught that by her father, Yūichirō Sawamura, before he passed away during the war. Umi does that because she believes that she must keep a memory of her father. After her father passed, Umi moved to Coquelicot Manor, a boarding house belonging to her grandparents. Her grandfather build a mast for Umi to hoist her flags there.
At the end of the movie, after Umi and Shun learn about their respective fathers from an old friend, these same flags can also be seen on the flagpost of Shun's dad's tug, most likely as a message to the departing ship and the captain.
Umi's flag signal in the paper[]
Shun's poem about Umi
In the beginning of the movie, Umi's friends hand her the school paper, Latin Weekly, and tell her someone has written a poem about her. The poem is accompanied by a drawing of the same flag combination she puts up every morning. At the time, it is unclear who the poem is from, but later on, Umi deduces that Shun Kazama must have been the writer.
The words translate to:
Fair girl,
Why do you send your thoughts to the sky?
The wind carries them aloft to mingle with the crows.
Trimmed with blue, your flags fly again today.
Shun's flag signal[]
Shun's flag signal
In response to Umi's message that he can see from his dad's tug, Shun raises three flags. He copies the original flags that Umi raises, with the aditional one meaning 'MER'. Altogether, the flags read 'UW MER', which can be translated as 'I wish you a safe journey, Meru' - Meru being Umi's nickname.
This combination of flags also reappears in a painting by Sachiko Hirokōji, another boarder in the house. She explains to Umi that she saw the flags on a tug that frequently passes by, and that they are an answer to the girl's own message. Later, Shun explains to Umi that he can see her flagpost from his dad's tug.
Miki's flag signal[]
Flags spelling H-O-K-U-T
In honour of fellow boarder Miki Hokuto's graduation and her becoming a doctor, a farewell party is held at the manor. Umi raises nautical flags meant to be spelling her name. As she very likely only has one set of flags, and flags are usually not repeated anyway, they spell H-O-K-U-T.
Shun reads the flags and informs the audience of their meaning, pleasantly surprising both his friend, Shirou Mizunuma, and Umi.