You can entangle an item with a shulker box, store that item while it's not a shulker box inside a shulker box, and use the quantum spanner to turn that item into a shulker box, which gives you a shulker box inside a shulker box, which is not possible in vanilla because you can store infinite items inside a single shulker box if you do that, crashing the game.
You can entangle an item with a shulker box, store that item while it's not a shulker box inside a shulker box, and use the quantum spanner to turn that item into a shulker box, which gives you a shulker box inside a shulker box, which is not possible in vanilla because you can store infinite items inside a single shulker box if you do that, crashing the game.