Each piece is allows access to the sale of a second unique NFT-Bonus. For example: If you buy #9, you and only you will have access to the sale of #9-Bonus. (nothing obliges you to buy #9-Bonus but think about it...) #9-Bonus is an alternative continuation of the image #9. And only you can see this alternative content ! 

The NFT-Bonuses combined build THE HIDEN PUZZLE together.

The NFT #3, #6, #9... etc are a bit special because they don't give access to one but two NFT-Bonus