You can build complex assemblies consisting of many components, which can be parts or other assemblies, called subassemblies. For most operations, the behavior of components is the same for both types. Adding a component to an assembly creates a link between the assembly and the component. When SOLIDWORKS opens the assembly, it finds the component file to show it in the assembly. Changes in the component are automatically reflected in the assembly.
The document name extension for assemblies is .sldasm.