Assemblies / End Item
Assemblies and end items are collections of parts, assembled or bundled in someway. Each are created with basic meta-information, however the main information is the 'Bill of Materials'. The BOM specifies what parts and sub-assemblies define the product and the amounts of these parts and sub-assemblies.
End Items differ slightly as they are normally used to define products that are being sold, so the highest level in the BOM, however this is not necessary. End-item differ from assemblies from having the ability to attach a serial number to the item, so you can link physical attributes to your digital representation. These BOM's with qty 0 can be kept as an archive of how the product is assembled.