My guess is that it's intended to be connected to a Cadmium Sulfide (CdS) photoresistor, to ensure that the relay is triggered only under certain light conditions (probably when it's dark).
I wouldn't be able to answer that question for you, since the datasheet isn't very helpful. You'll have to try it out for yourself. I would try:
a) Leave CDS floating (disconnected)
b) If that doesn't work, try tying CDS to VDD
c) If that doesn't work, try tying CDS to GND