In Demon SlayerDemon Slayer anime tilluntil the Entertainment districtDistrict arc, I beleivedbelieved that only Tanjiro can have marks, but in the Swordsmith villageVillage anime arc, it is shown that both Muichiro and Mitsuri getsget demon slayer marks but. However, in anime, it's not explained about the marks.
So, in which arc the marks will be explained?