From the A300-600 MM, Chapter 34-12-00-0:
B. Principle of Operation
(1)Barometric pressure display
The barometric display module enables the barometric pressure correction value to be manually displayed on two counters, in millibars and in inches of mercury, through BARO set knob. This value is transmitted to both ADCs, in sine and cosine resolver voltage form, where it is used as reference pressure for Baro Corrected Altitude processing. When BARO set knob is pulled out, a blue flag marked STD covers the two counters and both ADCs process Standard Altitude using 1013.25 mb as reference pressure.
The input from the Altimeters to the ADCs is shown on the system schematics, also.