Hello,
the hidden type form the XLSForm specification does not work (not implemented?) in KoBoToolbox. Why? - Update: Problem seems only in Formbuilder!
http://xlsform.org/en/#question-types
I would suggest to mention this difference to the spec also in http://xlsform.org/en/ref-table/
I know that calculate ( or relevant = false() ) could be used as workaround…
Kind regards