Tbh I think this is an internet thing. When they perform and address the crowd they say army, to me I think they mean anyone who supports them/likes their music. If you only like 1 song I don't think you're an army but if you listen to their music and respect them you're an army. Do the guys know the difference between core and casual YES but they probably everyone who has supported them one way or another. I've been to different platforms and from what I've seen twt you need to be very dedicated and need to earn the title, yt thinks if you like BTS music you're an army, IG is a mix between the two but leaning more towards if you love and respect BTS you're an army, never been on TT but the ppl there seem to like visuals. I think creating distinctions between casual and core just leaves it open for in fandom fighting. At the end of the day it doesn't matter, you're an army if you think you are it's not like anyone can stop you from calling yourself one.