My guess would be Bass-to-Treble “Energy Curve”
Slide left = Flat, reference, studio sound (low “energy”)
Middle = balanced
Slide right = Bass boosted + treble sparkled (high “energy”)
Internally adjusts parametric EQ in real-time. Because not every song requires constant EQ and may make it even worse. Would be nice to change EQ “on air”: fast and easy.