These are items that drop in auto-stacking mode (except Pages of Tome of Satanic Portal). When you pick up any stack item, it will show "Quantity", and when you pick up another item of the same type, the quantities will be added together and it will not take up inventory space!

To extract, just place it alone in the cube and it will extract 2 at a time.
To merge two stack items, just place one on top of the other.
To create a stack, just place any amount of the same item in the cube and it will be added together.

Practical example:

Attention: It will not be possible to recover the transformed item! Use the recipes below only when you are sure. These recipes are only meant for storing items that you can only carry one of, without needing multiple mules.