If you watch some videos from china, you may see that during fights, a little picture will pop up of a tool/treasure type thing (most common one I see looks like a piece of rope) which then gives stat boosts to the ninjas. I believe these treasures are what will go there. Beyond the fact that they automatically activate on certain turns of the battle and give unknown stat boosts, I don't know much more.
That is a different system that will be added soon to english version its not a tool that goes in that slot currently that slot is still locked in china and it seems it will stayed locked for a while.