The frequency balance of any audio changes at different volumes because of how our ears work, these equal loudness curves let you shape a sound to make it sound how it does at louder and quieter volumes without changing its character.
These curves were created from the ISO226 curves (updated version of Fletcher-Munson), specifically 1 base unit (bU) is the difference between the 44 phon and 40 phon ISO226 curves. I bought Slick EQ M specifically for its equal loudness curve, it works well but it's really only meant for small adjustments. I ended up using it for creative purposes and needed so many instances where it got to the point that even with Eco mode turned on it required more CPU than I had available, so this is why I made these. There isn't any real difference between the 50bU curve and 50 instances of the 1bU curve, but it saves a lot of CPU. Special thanks to this guy, you can also check out my matlab session here to see how the data for these curves was generated