Narrowing down my results works similarly. For instance: I click New selection, choose Sample Inc. in the field 'Client' and press the Search button. After the results have come up I decide that I want to refine my search. I go back to the search dialog box and click Narrow. In the field 'document type' I choose correspondence and click the Search button. Now the system returns only the correspondence for Sample Inc. (All other document types like invoices, delivery notes, etc. are ignored.)