Square Megaton

It names a unit of one million tons. In nuclear writing, the unit usually measures explosive yield as equivalent to one million tons of TNT, a common explosive used as a comparison standard. Nearby wording tells you whether mass or yield is meant.

Why is ㍌ in a square?

The square lets the long unit spelling fit one Japanese text cell. Japanese typography commonly gives characters square cells, so this compact sign takes one cell instead of the four used by メガトン.

Unicode compatibility normalization, a text cleanup that removes some presentation distinctions, can expand ㍌ to メガトン. The result is four katakana characters, so the compact one-cell form is no longer preserved.