When adding a custom sublist using a Saved Search in NetSuite, the Saved Search field that joins the search results to the record is "A field in the Available Filters subtab that has a Label." This field acts as a connector, linking the Saved Search results to the specific record based on the criteria defined. By setting up a labeled field in the Available Filters subtab, administrators can ensure that the custom sublist displays relevant data related to the record in question, enhancing the record's detail page with dynamic, search-based content.References: NetSuite Help Center, "Adding Sublists to Records"