The S2 pin cannot be left floating. The S2 pin can be utilized as either an active-low SUSPEND pin (in which state selected outputs are disabled) or as a frequency select control pin. If selected as #SUSPEND, pin 15 should be HIGH for normal operation and LOW to activate SUSPEND mode. If used as a frequency select, it can allow the user to toggle output frequencies. If no frequency select is needed, the CyberClocks software selects S2=LOW as the default setting, so S2 can be set LOW.
So by default, the recommended way to deal with pin 15 is tying the pin LOW if it is not being used for SUSPEND mode or frequency selection.