Entering custom properties for assemblies with many sub-assemblies and parts can be time-consuming. An efficient approach is to add the custom properties (part numbers, descriptions, and so on) in SolidWorks. Then check in the top-level assembly. You are prompted to enter the information for any documents that do not yet have the custom properties filled in.
You can use the SolidWorks API to generate custom properties for large numbers of documents; for example, to serialize document numbers or to standardize on a format of all caps.