What ㆛ means
It marks the third level in a kanbun return sequence.
Kanbun is Classical Chinese read with Japanese grammar. Kunten are guides to reorder it. The Japanese return mark 返り点 (kaeriten) sends readers back; ㆛ marks the 丙 tier.
The oldest Japanese evidence cited for return marks dates to 788; the sequence may come from a Silla example (Silla was a Korean kingdom). Before 1912, Japanese schools used different kunten conventions. Similar annotation practices appear in Korea, Vietnam, and China.
Specialist; it signals a text annotated for Japanese kanbun reading.
Don't confuse it with
1Questions people ask
1Why does ㆛ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ㆛
㆛\319BCodes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.