The backing is just chipboard. It's designed where your dies would face the outside and be held under the clips, so you can see them in the case when it's closed. Here's a link to a video if that helps better explain if I'm not finding the best words to paint the picture: https://youtu.be/iPgLdkbnDFM
I like to store mine facing the inside along side the stamp set. I also create a jacket to slide into the plastic sleeve with the set name and images of what's inside, so I can quickly scan my bookshelf for what I need. I hope that makes sense. I have over 100 of these now, and I love them for so many reasons.