Inmate number

Somebody answered this in the comments so I'm just repeating what they said. Inmate number is like their name as prisoners. So MC not knowing Hyeonjae's Inmate number feels like he don't know his prison name. And since Hyeonjae is mentally challenged, he's hurt about it. That's why he's acting weird around MC the past few chapters (not like he's ever normal but yeah)

Thanks for your explanation. I appreciate it!