The Demon Slayer show has these inscriptions on eyes that mean "upper 6" or "lower 12" etc. I haven't seen these "upper" and "lower" rank words in my studies. And the number symbols are unfamiliar too. It seems Tanjiro says "じょうげんのろく"
The eyes seem to spell out:
Looking at Jisho for that with a space yields a clue. Apparently 陸 is also 6 just like ろく or 六. Even though the main definition is land or shore.
Luckily 上弦 on jisho fits what I hear as "じょうげん" which means "1st quarter of moon; crescent moon" so I guess that means like a moon rank or something?
Why are these numbers written so differently in demon eyes? Is the literal translation more like "6 of the moon" or is this moon thing a standard ranking system? What's going on?