I am not sure if the free night certificate draws from a different pool of availability or not? My observation seems to suggest that it does. I had to rework nights at the IC Le Moana Bora Bora. I wanted to replace the second night from points to free night certificate. The point booking calendar view on Ihg showed no availability for the second night with 3 different logins , me, wife, daughter . However, clicking on the free night certificate and my daughters account allowed for booking a night for that second night. I was then able to cancel my point booking for the second night. This seems to suggest that even if there are not rooms available for booking by points, they still may be bookable with the free night certificate.