Backpacks

**Backpacks** are best understood as an extension of the inventory of Albert West. At the beginning of the game Dr. West has no backpack equipped and can only carry items that are directly equipped, including quick item slots. Once any backpack is picked up, Dr. West inventory appears and can be improved by switching over to a better empty backpack. Carried backpack can't be unequipped or thrown onto the ground.

Only empty backpack which is not carried can be switched over to, thrown onto the ground or stashed away into other container. When thrown onto the ground, an empty backpack can be used as a crate to store items/loot. It can only be picked up by picking up its contents first. A non-empty backpack can't be stashed into other containers, including metro train compartment.

## Inventory

The inventory is divided into six categories: Equipment, Medical Supplies, Food/Drink, Miscellaneous, Materials, and Data/Keycard/Facility. Items are automatically placed into their appropriate categories and cannot be manually moved to incorrect categories. These categories only exist in West's inventory; items on the ground are not categorized in any way, and even dropped backpacks will not categorize their contents in any way. However, items can be manually reordered by clicking and dragging them (there is no known way to do this with a controller, besides taking items in and out constantly).

### Container Limits

All containers have limits regarding how many items they can hold, and this includes all forms of backpacks. Containers have two limitations: a weight limit and a slot limit. The weight limit requires that the sum of the weights of all items in the inventory be lower than the container's weight limit. The slot limit restricts how many *stacks* of items can be in the inventory at once. Both limits must be maintained; if picking up an item would exceed either the slot *or* weight limit, then it cannot be picked up (although it can be directly equipped, if desired). With regards to West's own inventory, equipped items, including the quick item slots, do not contribute to either the slot limit nor the weight limit.

While West is not facing an interactable object, pressing the Interact button will open the "ground inventory", which simply shows what items are on the ground nearby. Unlike traditional containers, the ground inventory has no weight or slot limits. However, if an item is dropped onto the ground, then it is both visible and has collision. Since items cannot physically overlap, the ground does have a limit to how many items it can hold, in the form of how much physical, walkable space is available to drop items onto.

### Item Stacks

Every item has a stack limit. About half of these items have a stack limit of only 1, most being equipment of some sort. Others have varying stack limits, ranging from 2 to 999 in the most extreme cases, although most are in the range of 5 to 50. If an item has a stack limit higher than 1, then picking it up while also carrying another copy of the item will cause both copies to occupy the same slot, thus creating a stack. Although this allows them to use only 1 slot, their weights still accumulate appropriately, so a stack of 5 items will weigh 5 times as much as a stack of 1 of that item. Once the stack reaches the item's stack limit, picking up more copies of the item will create a new stack, which will occupy a second item slot.

For example, say you have 1 basic medical kit in your inventory, which has a stack limit of 5. It weighs 0.50 kg and takes up one item slot. If you find another one and loot it, then it will stack with the already-held one. You now have a stack of 2 basic medical kits, weighing 1.00 kg and taking up one slot. If you loot 3 more, you will have a stack of 5 kits, weighing 2.50 kg and taking up one slot. If you loot 1 more kit, you will have one stack of 5 kits and another stack of 1 kit, weighing 3.00 kg and taking up two slots in total.

An item's stack limit is also used for engineering items at the Bio Combine System or 3D Printing System and producing items at the Mine Extractor and the Biosphere Accelerator. The queues in these machines work similarly to the item slots in inventories, so that multiples of an item can be queued and only occupy one slot in the queue. Notably, up to 50 of an item from the Biosphere or Mine can be queued while only occupying one queue slot, but to queue up 51 (for example) would take two queue slots.