Square HP

It stands for horsepower; the square is a compact typographic form.

This legacy character belongs to a Japanese and broader East Asian text-encoding tradition: it let the Latin letters HP occupy one full-width cell, about as wide as an East Asian character. The horsepower unit dates to the late 18th century, when it compared steam engines with draught horses.

If ㏋ displays as an empty box, the font lacks this character. Unicode compatibility normalization, a text-cleanup process called NFKC or NFKD, can turn it into plain HP and remove the box.