Johnson Controls GLAS Installation Instructions Manual page 6

Terminal descriptions
O/B Heat pump reversing valve (supports both O type and B type)
AUX
Auxiliary - humidifier, dehumidifier, or ventilator
Rc 24 VAC Cool
Rh 24 VAC Heat
C
24 VAC Common
GLAS smart thermostat installation instructions - 2018 Johnson Controls
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