Little about Asano Mikawa Shiro Nagura 

They are from Junmikawashiro minein Aichi prefecture:

There are 4 stratas
from finest to coarsest
Koma, Mejiro, Tenjyou, Botan
コマ ( 細 ) 、目白 ( めじろ ), 天上 ( てんじょう ) 、ボタン、
And 4 grades (colours)
Superior high grade, High grade, Superior selected grade, Superior grade

Koma is most fine in grit but Botan, Tenjyou and Mejiri is fine too and its not much difference for knife sharpening

Honing a Kamisori with Asano Naguras, Iwasaki, Nakayama