An inventory assembly item is one of the types of line items you can use when you are filling out a sales or purchase form. An assembly item lets you combine inventory part items and other assembly items (subassemblies) into a single item by defining a Bill of Materials, which lists the assembly item's component parts. You can also include the costs associated with building the assembly item by adding non-inventory part items, service items, and "other charge" items to the Bill of Materials.
Creating and adding assembly items to inventory is a two-step process. You first define the Bill of Materials that specifies what components go into making the assembly item, and then you build a certain quantity of the assembly item so QuickBooks can deduct the component parts from inventory and add new quantities of the assembled item. In other words, as soon as you build an assembly item, its component parts no longer exist as separate parts in inventory because they have become part of a new inventory item called an assembly.