Systainers are the modular storage cases produced by Tanos and used by Festool, Makita, Triton and other tool manufacturers. The cases feature two little recessed panels on the front, indicated with the arrows below:
You can slide some cardstock or a piece of paper into those recesses, as a way of labeling what's in the case.
However, Tanos has also designed these ADAPT-L adapters that can be slid in there instead, for extra functionality.
They allow you to clip in this box for holding small parts…
…or carry longer items that don't fit within the cases.
It's a clever use of the labeling space. I bet if Tanos released 3D files, we'd see people coming up with all sorts of 3D-printed accessories.