This setting is found under VPS: Image Field Settings and is is optional. It allows you to specify custom fields from the image page to display.
Please note that not all Fieldtypes are supported. Supported fields are of the types: 'FieldtypeDatetime', 'FieldtypeEmail', 'FieldtypeFile', 'FieldtypeFloat', 'FieldtypeInteger', 'FieldtypeOptions', 'FieldtypePage', 'FieldtypePageTitle', 'FieldtypePageTitleLanguage', 'FieldtypeText', 'FieldtypeTextLanguage' and 'FieldtypeURL'.
Image fields are not supported, the assumption being if the field is being used as a 'one-page-per-image' strategy, no other images outside the ones already being used in the field would be required.
Please note that this setting only applies to Thumbs View (specifically, Thumbs View: Table).