The document discusses the evolution of the NAL fitting formula from a linear to a compression formula with three gain targets. The research showed that for gently sloping losses, similar gain was needed at 65dB inputs. NAL aims to equalize loudness across frequencies rather than normalize to unaided levels. This preserves speech spectrum loudness relationships and is theorized to maximize speech understanding. NAL gain targets are calculated based on phonemic information from the Articulation Index. The two key features of NAL are equalizing loudness across frequencies and providing less gain where loss is worst.