The functions screen-monitor-geometries, screen-internal-geometries and pane-screen-internal-geometry support the notions of monitor geometry (which includes "system" areas such as the Mac OS X menu bar and the Microsoft Windows task bar) and internal geometry (which excludes the system areas).
Note that code which relies on the position of a window should not assume that a window is located where it has just been programmatically displayed, but should query the current position by top-level-interface-geometry. This is because the geometry includes system areas where CAPI windows cannot be displayed.
CAPI supports multiple monitors by providing functions such as screen-internal-geometries to query "screen rectangles" representing the area of each monitor. The function virtual-screen-geometry returns a rectangle just enclosing all the screen rectangles.
There is a "primary monitor" which displays any system areas. The origin of the coordinate system (as returned by top-level-interface-geometry and screen-internal-geometry) is the topmost/leftmost visible pixel of the primary monitor. Thus (0,0) may be in a system area such as the Mac OS X menu bar.
You can specify that that the geometry of a top level interface should be saved when the interface is closed and be used to define the geometry of the interface when it is opened again (potentially in a different invocation of the application). You need to define a method of top-level-interface-save-geometry-p that returns true for the interface class. You normally also need to specify where to save the geometry, using top-level-interface-geometry-key.
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017