An assembly item is made of other items, such as a "gift basket" or a "complete PC". Before you can sell it, it must be assembled: the system removes the components from stock and adds the assembled item.
Create the item with type Assembly and list its components and their quantities on the item card.
Open the Assembly / Disassembly screen.
On the other side, the system lists the components required — needed and available quantity of each — and warns you if there isn't enough.
Click Save to post the assembly.