The board's two 3.3 V outputs (3.3 V header pin output & STEMMA QT 3.3 V output) can be enabled/disabled independently for power saving.In the photos below, the red LED is powered from the header pin 3.3 V output, and the STEMMA QT module from the STEMMA QT 3.3 V output. Switching on/off the controls on the dashboard, these two devices turn on/off accordingly:

Header pin 3.3 V output enabled, red LED on

Header pin 3.3 V output disabled, red LED off

STEMMA QT 3.3 V output disabled, green LED on STEMMA QT module on

STEMMA QT 3.3 V output disabled, green LED on STEMMA QT module off