Add a Blender object of type "Empty" to your scene. Add a property to the Empty object named group_terrain, group_beaches, group_shoulders, group_taxiways, group_runways, group_markings, group_airports, group_roads, group_objects, group_light_objects or group_cars. Give the property a value between -1 and -5 to make X-Plane draw your object before this group, 0 to draw your object with this group, or between 1 and 5 to draw your object after this group.