It is an absolutely moronic and insanely idiotic rule Hyatt has that serves only 1 purpose, and that is to increase breakage in the most customer unfriendly way possible. I have personally battled it many a time. Just an arbitrary, you can't use THIS free night type but THIS one over here, ie points, is fine. They are otherwise totally fungible...same requirements for available room types, etc, but you randomly can't use the certs with the DSU. I always just book the stay without the cert, apply the upgrade then book the cert night at the end or beginning KNOWING that technically the hotel can ask me to change rooms for that one night at the end or beginning, which has never happened once. Every rep I talk to argues and fights me when I try and apply a DSU where there is a cert night after or before, so I just cancel it and then tell them, apply the DSU now. They always piss and moan but they do it, and then I just rebook the free night myself after. A completely terrible rule that serves zero purpose, can always be easily circumvented and just makes the customer's life harder for zero reason. The exact opposite of what any loyalty program or any business for that matter should ever be doing to customers.