by Dominique Bauer.
Thank you Mary and Ken for your replies.
Moodle 3.4.2.
Maybe I do not do it in the right way. Let's say I have a course with x topics, say 20. I want to create a new course with 5 of these topics. So I backup the 5 topics and restore them in a new course. The new course includes the 5 topics but also 15 empty topics.
Not a big problem, just a little annoying because I have to delete the empty topics.