typedef void (* XPLMMapLabelDrawingCallback_f)(
                         XPLMMapLayerID       inLayer,    
                         const float *        inMapBoundsLeftTopRightBottom,    
                         float                zoomRatio,    
                         float                mapUnitsPerUserInterfaceUnit,    
                         XPLMMapStyle         mapStyle,    
                         XPLMMapProjectionID  projection,    
                         void *               inRefcon);    

This is the label drawing callback that enables plugin-created map layers to draw text labels using X-Plane’s built-in labeling functionality. You can request an arbitrary number of text labels to be drawn via XPLMDrawMapLabel() from within this callback, but you may not perform any OpenGL drawing here.

Labels enqueued by this function will appear above all OpenGL drawing (performed by your optional XPLMMapDrawingCallback_f), and above all built-in map icons and labels of the same layer type (“fill” or “markings,” as determined by the XPLMMapLayerType in your XPLMCreateMapLayer_t). Note, however, that the relative ordering between the drawing callbacks of different plugins is not guaranteed.