발령 (발 = issue/emit, 령 = command/instruction) is an official instruction or order of sending someone to a post. For important jobs, there is usually a written notice bearing a signature of the person in charge. We typically say 발령이 나다 or 발령나다, where 나다 means something is issued or an event comes about, as in 부산으로 발령이 났다 (I was posted to Busan), 영업부로 발령났다 (I was assigned to Sales), etc. (발령하다 or 발령(을) 내리다 would be issuing or ordering such a thing, as the department head would). 발령난 is an adjective form of 발령나다 used to modify a noun, like "assigned / appointed". E.g. 오지로 발령난 (or 발령이 난) 친구는 불만이 컸다 (My friend who got posted to a remote place was very unhappy about it), 제가 인사부로 발령난 것을 어떻게 아셨어요? (How did you know I got assigned to the Human Resources Department?).

@tony 감사합니다! This is such an amazing explanation ❤️