Our help manual is part of this community web site. You can find it here: How To - Agenda Community
There are good intro videos here: Video Tutorials
It’s difficult to answer your question, because I don’t know the exact context. Can you copy the text you saw on our web site? I can then give a good answer.
Agenda is not a calendar app, so it doesn’t really have the concept of a due event. What you can do is link a note to a calendar event. You can also set a date range for a note using the calendar control top-right on each note. You just drag across the dates you want, or use SHIFT-click on the Mac to select a range.