Some questions about AD7682

Question 1:

The AD7682 datasheet describes that there is a pin called COM which can connect to 0V or VREF/2V. But in which case we should connect the COM pin to the 0V and in which case connect to VREF/2V? What's the main function of the COM pin?

Question 2:

The AD7682 has REF pin and REFIN pin. The datasheet describes that if connect the REFIN to the reference voltage. The REF pin will produce a buffered reference voltage that comes from the REF pin. Would the ADC working if i connect the REFIN pin to the reference voltage and hang up the REF pin?