Full-width cm

It means centimetres in one full-width character cell. The square is a layout container, not a claim that the unit is square centimetres. Unicode groups it with square Latin abbreviations designed to occupy one Chinese, Japanese, or Korean character position beside ideographs.

Japanese layout guidance uses ordinary cm in horizontal text and commonly uses katakana, a Japanese syllabic script, for unit words vertically. A full-cell character suits grid-oriented layouts. Typography can also fit ordinary Latin letters into a full-width cell with styling, so ㎝ is one compatibility option for that layout.

Where did ㎝ come from?

It comes from legacy East Asian character sets. Square Latin abbreviations mostly came from Korean legacy standards; the wider set also draws on Chinese, Japanese, and other East Asian standards.

㎜ and ㎞ use the same full-cell strategy for millimetres and kilometres.