What ㋌ means
Hg means mercury; the box sets the character's width.
The square let Latin abbreviations fill a character cell the width of an East Asian ideograph. Older Korean character sets are a major source of this layout convention.
Encoding records reach 1999 Korean character-set proposals; a 2001 proposal assigns provisional code points to square Hg, erg, and eV. Their frames preserve layout, not new chemistry meanings.
Technical and legacy: the box marks a one-cell layout form of the chemical symbol `Hg`.
NFKC drops the box; use plain `Hg` where chemistry data must retain its standard form.
Don't confuse it with
4Questions people ask
1Why does ㋌ become Hg?
It can: NFKC, a compatibility-normalization process, turns ㋌ into `Hg` and drops the square. Preserve the original when that one-cell appearance matters.
How to type ㋌
㋌\32CCCodes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.