Hi Peter,
I do agree that understanding the nature (and use) of the 3 questionnaire types available can be confusing. When I used to maintain the Questionnaire plugin, a few years ago, I spent a lot of time writing (and re-writing) the documentation. I did my best but there may be room for improvement in the description available here: https://docs.moodle.org/39/en/Setting_up_a_questionnaire#Questionnaire_Type
