XPLMWindowPositionMode describes how X-Plane will position your window on the user’s screen. X-Plane will maintain this positioning mode even as the user resizes their window or adds/removes full-screen monitors.

Positioning mode can only be set for “modern” windows (that is, windows created using XPLMCreateWindowEx() and compiled against the XPLM300 SDK). Windows created using the deprecated XPLMCreateWindow(), or windows compiled against a pre-XPLM300 version of the SDK will simply get the “free” positioning mode.

xplm_WindowPositionFree"0" The default positioning mode. Set the window geometry and its future position will be determined by its window gravity, resizing limits, and user interactions.
xplm_WindowCenterOnMonitor"1" Keep the window centered on the monitor you specify
xplm_WindowFullScreenOnMonitor"2" Keep the window full screen on the monitor you specify
xplm_WindowFullScreenOnAllMonitors"3" Like gui_window_full_screen_on_monitor, but stretches over *all* monitors and popout windows. This is an obscure one... unless you have a very good reason to need it, you probably don't!
xplm_WindowPopOut"4" A first-class window in the operating system, completely separate from the X-Plane window(s)
xplm_WindowVR"5" A floating window visible on the VR headset