Why is container "relative" and box "absolute" and not vice versa?

Question

JS lesson: DOM & events > Animation:

I don't understand the positioning here. Shouldn't the green container have an absolute position on the page and the red box be relative to the container? Why is it the other way around?
Can someone explain?

Answer 1

Both can be relative or absolute , it doesn't matter. red box always stay inside the green container

Answer 2

Always take relative position on the parent or man term

Answer 3

I quote Lavish, don't know why the lesson specifies that as requirement