If you're new to USC, you might want to check out the Step-by-Step Tutorial, too!
Inventory
General
Each commando has an inventory space of a total of nine (9) items / item stacks. This space cannot be extended or upgraded, so you'll have to plan carefully with your mission loadout. The inventory space of 9 slots includes the items you equip for use (e.g. weapons), i.e. the inventory is not your “backpack”, but your total item carrying capacity. The silver lining is, in USC you don't have to worry about the weight or size of items.
The inventory is automatically visible on the Squad Assembly screen and can be accessed at any time while in-mission without any AP cost (or other consequence).
To access the Inventory panel while in-mission, just left-click on the main weapon icon under the portraits, or simply press the “I” key (the same goes for closing it). Your current action (if any) will not be interrupted until you actually do something in the inventory.
Equipping items
Any kind of interaction (except placement) with items is performed by right-clicking: if the item can be equipped, right-clicking on it equips it. If there's an AP cost to equip (or use) an item, it is displayed in green text in it's slot's top left corner.
There are two types of items that can be equipped:
(1) Weapons: you may carry as many weapons as you see fit, but only one can be equipped at a time. The currently equipped weapon is shown on a red background. Its name and icon are also visible above the Inventory panel, along with information about current and available ammo. A weapon must be equipped at all times. You can unequip a weapon by equipping another one. It costs 5 or 3 (if skill is at 100%) Action Points.
(2) Modules: items that enhance your weapons or armor often also need to be equipped (activated). These are displayed on a green background when equipped.
Equipped items can be moved in the actual unit's inventory, but cannot be put down or passed while equipped!
Moving items
Moving an item (or a stack of items) is done very simply: first, left-click on the item to move, then left-click again where you want to move the item to. The destination can be either an empty inventory slot, an accessible map grid (with no other item on it), or another friendly unit if you're standing next to it. In Squad Assembly, items can be passed between units by clicking on the target unit's portrait while the item is selected to move. Putting down or passing items cost 2 APs! If you don't want to move an item you selected, just left-click on it again.
Items lying around on the map can be picked up similarly, but you need the Inventory panel to be open! Or, you may hold down the “X” key when left-clicking on the grid. You can only pick up an item if you have an empty slot. Picking up an item also costs 2 APs.
Item stacks
Some items are stackable, which means that you can store more of them on the same single slot/grid. Stackable items always (even if there's only one of them) have their quantity displayed in the bottom right corner of their slot. Exact same types of stackable items can be put together by simply placing one upon the other. (And they can be picked up with no empty slots left if there's a slot with the same item.)
To divide item stacks, first select the stack (left-click on it), then right-click on an empty slot. The divide stack dialog will appear, where you can set the exact quantity of the new stack. This does not cost any APs.
Any kind of ammunition is stackable. Other stackables include stimulants, grenades, and crafting materials. The flag “stackable” is displayed for these items in the item list when “shopping”.
Stacks do NOT have a quantity restriction–your budget will deal with that 
.
Using items
Use any item that has a usable function by right-clicking on it. This will instantly consume consumables (1 from a stack), or activate the item so you may select a target for the action. Activated items are shown on an orange background. Select the target by left-clicking on the desired (and available) location. To abort using such an item, simply right-click on it again or close the Inventory.
Working with weapons
Weapons can be loaded and unloaded using the Inventory (in addition to main weapon controls). To (re)load a weapon, select ammo by left-clicking on it, then left-click on the weapon. If there's a different subtype of ammo currently loaded, you'll need an empty slot available where that ammo can unload. Loading the weapon this way will cost the corresponding APs (see Weapons).
You can unload a weapon by left-clicking on it, then clicking the “Unload” button below the Inventory. You'll need an empty slot or an ammo stack of the exact same subtype. It will also cost the corresponding APs.
Miscellaneous
Crafted items have a * (star) symbol in the top right corner of their slot.
By hovering the mouse cursor over an item in the Inventory, you can get additional info in a floating window about the name, type, value, and basic function of the item.
When the Inventory is open, there's a button called “Lights” below it, which toggles the unit's headlights on/off. This can be useful mainly in PvP games to avoid being detected by raw sight.
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