I often use Moi to model custom boxes to keep things in.
My usual approach is to create two rectangles (Closed curves) representing the outside and inside edges of the box, select both and planar the space between them. This planar surface is then extruded upwards to make the sides. I then select the outer of the two closed curves and extrude it down (Cap Both Ends) to create the base.
Then I often want to move the outer closed curve (which I have extruded to create the base) to the top of the box to help make the lid.
If I select just the closed curve and move it up, the extruded base moves with it - even though I have not selected it. This happens even if I hide the base before selecting and moving the curve.
If I Boolean Union the sides to the base to create one solid, then the closed curve moves alone.
Why does moving the selected closed curve cause the solid extruded from it to also move?
I attach a simple version to illustrate.