This is the place where the inform for users who email in, and are not a JIRA user is recorded.
To access this section, click on Email only users, on the left of your screen. Then select Edit to bring up the edit screen.
Select the appropriate custom fields from the drops down. Note, the custom fields will need to have been created. The field type is listed below each selection box. Once the fields have been set, click on Submit to save your changes.