Compartments are basically upgrades with some usefull effects for your backpack.
Current compartments:
Basic: The basic compartment has no effect. It’s only used to craft other compartments. You can craft it with one obsidian leather.
Soulbound: If you die while having a backpack with this compartment in your inventory you will keep the backpack with all items in it. Needs one nether star.
Pickup: Picked up items go automatically in the upgraded backpack if they are already at least once in it. For example if your backpack contains 1 redstone and you pick up another redstone the redstone will go in the backpack instead of in your inventory. Needs one hopper.
Empty: Shift-Rightclick an inventory block to move everything from the upgraded backpack in the inventory block. Needs one cauldron.
Infinite: Allows you to put other backpacks in the upgraded backpack. This can lead to an infinite storage system. Needs one diamond block.
Incinerate: Turns your backpack in a portable trash can. Items in the backpack will get destroyed immediately. Be careful with this compartment! Needs one lava bucket.
Armored: Gives the upgraded backpack a higher protection value (3 Armor Points instead of 1/2 Armor Point). Needs an iron chestplate.
Swap: Press the ‘Swap Hotbar’ key (the default for it is ‘V’) while having the upgraded backpack in your inventory to swap all items of your hotbar with all items of the backpack’s first 9 slots. This doesn’t swap backpacks so it’s not recommended to have any backpacks in your hotbar while swaping. Needs one gold block.
Guarded: Players who are not owners of the backpack take damage and receive slowness if they try to open it. Owners of the upgraded backpack also receive a message that someone tried to open their backpack.
Quivered: Shooting with a bow, consumes arrows from the backpack instead of the inventory but you need at least one arrow in your inventory. Needs one leather.
Ender: Turns the backpack into a portable ender chest. Needs one ender chest.
Sending: Shift-Rightclick an inventory block to select a location. All items in the backpack will be sent to the inventory. You have to be in the same dimension or the inventory block has to be in the overworld (at the moment). Needs one sending controller which can be crafted with 4 ender eyes, 4 redstone blocks and 1 diamond block or nether star.
Creative: Items in the backpack will not get removed if you try to take them out. Cannot be crafted and can only be received with commands or within the creative tab!
Compartments: