What are the available fan modes on the T6 Pro thermostat?

The thermostat offers three fan modes: Auto (fan runs only when heating or cooling is active), On (fan is always running), and Circ (fan runs randomly about 33% of the time). To change the mode, press the Fan button to cycle through the available options and leave it on the desired mode to activate it. Available fan modes may vary with system settings. (Page 7)