Next time, please stick to your orginal thread for clarifications.
The dummy node approach for a reserved area assumes that you have exactly one reserved area. There is no way to reserve more than one such area.
How can assign some boxes fixed positions? For example, I would have entered the starting top left and bottom right of a target.
I am afraid I do not understand that question.
What are boxes? Do you mean nodes? If so, use "Layout" -> "Selection (Partial)".
What do you mean by entering "the starting top left and bottom right"?