Square percent sign

It means percent, exactly like %. Inside the square, in tiny stacked letters, is パーセント: “percent” written in katakana, the Japanese script used mainly for foreign loanwords. Japanese borrowed the English word, so 50㌫ reads as “50 percent”.

Packing a whole word into one character cell is a Japanese typographic habit for units and common terms: ㌔ for kilo, ㍍ for metre, ㌦ for dollar. The square fits the word into the same space as one kanji, which keeps tables tidy and stays upright in vertical text, where five separate letters would take up a whole column.

Where did this square character come from?

From Japanese computer character sets of the late 1970s and 1980s. JIS C 6226, Japan's 1978 standard for coding text on computers, left some rows empty. Before 1983 the computer maker NEC filled one of them with 82 extras, including circled numbers, square units and ㌫. Microsoft and other makers later adopted that row, Unicode took it in to stay compatible, and in 2000 the row also entered the Japanese standard JIS X 0213.

For years these “machine-dependent characters” were notorious: a ㌫ sent to a system using an incompatible encoding could arrive as a different symbol or as gibberish. Unicode text mostly fixes that, but a font without the glyph still shows a blank box, and some Japanese online forms, government application systems among them, still reject ㌫. There, % or パーセント is the safe choice.