On the mechanical browser, you can restructure the hierarchy of an assembly as needed.
You can place or create an assembly of components at a single level and subsequently adjust the assembly structure, grouping components into subassemblies for manufacturing, inventory, and other uses.
To restructure, drag components from one location in the mechanical browser hierarchy to another. However, you typically need more than one view to represent parts and assemblies, so restructuring the hierarchy of components alone is not sufficient. You must restructure the hierarchy of views as well. During the drag operation, AutoCAD Mechanical toolset displays the restructure components dialog box to where you can specify the hierarchy of component views.
The Component Restructure dialog box displays views of the component and the assembly so you can drag the component views to the assembly views. If you don't map all the components views to the views of the assembly, the component becomes a ghost component.
A ghost component is a component node on the mechanical browser that is in an intermediate state of restructure. Ghost components exist solely for the purpose of holding component views until they are mapped to the correct parent view. On the mechanical browser, ghost components are indicated by the 
icon.
In addition to an incomplete restructure operation, ghost components could occur as a result of an AutoCAD COPY, ARRAY, or MIRROR command as well. Ghost components prevent several operations from being executed and must be resolved as soon as they occur. For example, you cannot cut or copy subassemblies containing ghost components to the clipboard. To eliminate a ghost component, you must drag the ghost component views to the appropriate parent subassembly view.