You use ‘Reference’ if you want the imported column to be stored in an entity that is associated with your main entity, instead of in your main entity. Or as the AppstoreApp’s documentation phrases it:
Reference to import objects Sometimes you want each imported object to have a reference to one existing object. For example, you want each object to reference the FileObject it was created from. Set this field to the reference you want to fill this way. When you call the import Java action the selected reference will be set with the object which you can provide as a parameter in the action.
XPath is not used here. ExcelImporter will check for the referenced object by key:
I’ve found “Reference” only to work if the main entity doesn’t have the row populated. Is this by design?