When Yeongwonhi means "private arcs" she means that you shouldn't do something that is irrelevant to the plot. You can free form role-play in between major story arcs, but don't do things like determining another character's backstory unless approved by that character themselves.
*Private conversations between characters are allowed, training with other characters is allowed, but please keep these "mini-arcs" posts to a minimum. The story will get nowhere if everyone is at different places too often. We don't want characters suddenly running in all deus ex machina style...