The Custom format option allows users to customize their output based on the fields present in the documents they have selected. Custom formats can be selected when using the Download all results option or when selecting from the Export/Save options under the heading Save as file including PDF, RTF (Word), HTML, or Text only). Custom formats are also available when e-mailing or printing search results.
The field chooser list allows you to designate the fields you want to include in your output.
A custom report is automatically generated that includes only the fields you have selected in each record.
Source: ProQuest Dialog Frequently Asked Questions