When you when as much base trapping as possible and you’ve already done all the corners, flat on the front and back will always be the best, and you’ll notice that these traps are spaced several inches off the wall, which makes them absorb to an even lower frequency. These RFZ panels on these particular places on the side handle the early reflections from the side walls. The ceiling, of course, is already 100% absorbent, so there are no reflections.
Reflection control is the other half of the equation. Base trapping as much as you possibly manage is always needed and useful, but these side wall reflections are important. So, these are key places called reflection points that really need absorption. Some people like diffusion at the side wall reflection points, but I really prefer absorption. I’d love to sell people more diffusors, but in practice the RFZ panel is really about the best thing you’re going to put there.