To enable customers to refine product search results by a custom attribute like weightCapacity, the developer must first define this attribute as a search refinement in the system's configuration. This involves setting up the attribute in the Business Manager under the search refinements settings for the applicable category or globally.
Once this setup is complete, rebuilding the product search index is necessary to apply these changes and ensure the search engine recognizes weightCapacity as a refinement attribute. This process updates the search index with the new configuration, allowing the attribute to be used as a filter in storefront search functionalities.