Right now there is a limitation of the Custom Rewards API that makes it so a client can only update the custom rewards (and their redemptions) that it has created. However, with EventSub we can receive notification of any Custom Reward, regardless of whether the client created it or not. This means you can write a client to take some action upon Custom Reward Redemption, link it up to a preexisting Custom Reward, and it will take action. But the Client will never be able to programmatically mark the Redemption as FULFILLED, because the client didn't create the Custom Reward but rather re-used an existing one.

This is an arbitrary requirement of the API that doesn't seem to be serving any real purpose. Let clients act on any Custom Reward!