It's 2019 and we still, as a board, want to shoehorn RPO as a single thing. It has so many variations now from under center, the gun, outside WRs, bubbles, the slot, etc., that it's actually a fascinating concept used at all levels of play. But, let's be absolutely clear, it's not exclusive to numbers, pre-nap, post-snap, or hanging defenders.
I'm about to ask some of our former board QBs to do a podcast with me. I see the RPO from the defensive side, but I know for sure this is an area of tremendous variation and would love someone who's actually executed it as a QB to discuss all the variations.