In Demon Slayer, what are the "upper" and "lower" rank words and numbers?


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:

陸上弦

Looking at [Jisho](https://jisho.org/search/%E9%99%B8%20%E4%B8%8A%E5%BC%A6)
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?

[![upper six demon eyes in demon
slayer](https://i.stack.imgur.com/G0jSn.png)](https://i.stack.imgur.com/G0jSn.png)

[![lower five moon demon
eyes](https://i.stack.imgur.com/y1oqO.png)](https://i.stack.imgur.com/y1oqO.png)

