The structure indicates which components make up the individual assemblies. Components, which form their own individual assembly (like, for example, "C"), are displayed in the same structure, almost in the form of an individual material BOM.

It is clear in this structure that a component can be both an item and a header material. As a header material, it forms its own assembly.

The structure of the subordinate assembly is indented directly behind this header material and is displayed when you enter a higher structure number.