That enables internal processing. The datasheet won’t explain that – you need Realtek’s application note.
Designing with the ALC271X family needs careful attention to power sequencing, grounding, clocking, and jack/mic wiring. Follow the datasheet exactly for recommended component values and timings; validate audio quality with measurements and tune driver settings for best results. alc271x datasheet
Often includes integrated headphone amplifiers with "jack-sensing" capabilities to automatically detect when a peripheral is plugged in. That enables internal processing
In HD Audio mode, this pin goes HIGH to turn on an external amplifier (usually a cheap Class-D amp for laptop speakers). In AC’97 mode, this pin is a GPIO. this pin is a GPIO.