I explained how there's a broad umbrella of communications and that many disciplines can be seen to overlap. I gave the standard answer that public relations is what others say about you and advertising is what you say about yourself. I don't think he was asking this to see how I thought or to compare my answers to others'. Because he admitted he didn't understanding marketing communications, I believe he was honestly confused about roles outside of technical sales and seeking to build his understanding of the role he was trying to fill.