When you create a subassembly in place, you define an empty group of components. The new subassembly automatically becomes the active assembly, and you can start to populate it with placed and in-place components. When you reactivate the parent assembly, the subassembly is treated as a single unit in the parent assembly.
Optionally, you can select components at the same assembly level in the browser, right-click, and then select Component Demote to place them into a new subassembly. You are asked to specify a new file name, template, location, and default bill of materials structure. You can then move components between assembly levels by dragging components in the browser.
Subassemblies can be nested many layers deep in a large assembly. By planning and building subassemblies, you can efficiently manage the construction of large assemblies. You can create subassemblies that match the intended manufacturing scheme to facilitate the creation of your assembly documentation.
Skill Builder: http://www.autodesk.com/inventorpro-skillbuilder