I like to think of the SE/PDF curve as the bow's intrinsic ability to store energy. It is far from rigorous but is a good way to think about it, especially, since we are so focused on draw weight as a key measure. We forget that force only tells you what it takes to move the string, but it is energy that our body must put into the bow. We will encounter different magnitudes of force along that journey, which is represented in the DFC. What that means is that it takes me more energy, at my long 32 inch draw length, to pull a bow to full draw at 40 pounds holding weight, than someone with a 28 inch draw length. So 40 pounds is harder for me than it is for someone with a shorter draw. On the flip side, I get more energy out of the bow, and consequently, more speed. So why do we mark limbs in pounds rather than energy? Because it is easier to measure and people understand pounds better than energy.