You can build complex assemblies consisting of many components, which can be parts or other assemblies, called subassemblies.
All large scale design uses complex assemblies. You may need to simplify complex assemblies to improve performance, including creating simplified configurations of the assemblies.
Large Assembly Mode offers system options that improve assembly performance. You can enable it at any time, or you can set Large Assembly Mode to turn on when the assembly reaches a specified limit. You can customize it and combine it with other simplification methods, such as toggling the visibility or changing the suppression state.
Large Design Review opens very large assemblies quickly, while still retaining capabilities that are useful when conducting design reviews of assemblies.
When you want to work on a small subset of components in a large assembly, you can improve assembly performance by using Selective Open to open a simplified representation of the assembly. You specify which components to load; other components are not loaded and not visible, but the effects of their mates are retained.
For more information about these methods, see SolidWorks Help > Assemblies.