The Field Sets subtab enables you to select regular event fields that are part of a field set using a functionally organized field selector similar to that in the CCE and active channel editor. You can also use field sets in the Field Sets tab to narrow the list of fields down to those you are interested in.
You can navigate the entire event and resource schema for the fields you are interested in, or select a field set from which you want to select fields in the Choose fields from drop-down menu.