The recommended workflow for the picture attached is to create the frame geometry from 3 different frames with no curtain wall panels between the horizontal ones. It is not recommended to refer to project coordinates from curtain wall frame subelements.
The reason why the curtain wall panels have access to these global variable values is that the 2D Symbol of the curtain wall panels can be drawn by the GDL script (the frame 2D symbol is always drawn by ARCHICAD). For proper panel thickness representation at the cutting plane at a slanted Curtain Wall the slant angle is needed for symbolic 2D at the panels.
Library Developer, Library Team