How do the x and y coordinates work with multi-monitor? (or percentage)

Does anyone know how the x and y positions work from 0x0 in multi monitor? Or the percentage?
I checked the docs but they do not mention it.
I guess on a single monitor it is easy, but my current setup is 2 x 3840x1600 on top of each other with the MacBook screen adjacent to the right of the bottom monitor.
So is upper top-left 0.0?
There is a bug (reported, I added my comment as well) that the "Move / Resize" action crashes when trying to change the x-pos and y-pos. So it is difficult to test :-(. And I want to "Move / Reszie" my windows....
I am using the Mission Control setting "Displays have separate Spaces" with multiple desktops defined.
image Current monitor setup.