About Empty Components in an Assembly
You can create a part or subassembly without any geometry and it appears in the Model Tree window. If you save the top-level assembly, the system saves this empty component also.
When you create a feature in an empty component in an assembly, you are put directly into Sketcher mode and are not prompted for sketcher references (or external references, in the case of datum geometry).
When you define empty components within the context of an assembly, the system places them in the default location of their parent assemblies (using the Default constraint type).
Once you have created an empty component within an assembly, you can create default datum planes by retrieving the empty part or subassembly into a separate window and clicking . You can also create features for the component.
When you create features that reference the assembly in an empty component, you can no longer redefine the placement of the component.
For empty parts or assemblies with no geometry, the Model Size parameter has a value of zero.