Qml Location Map visibleRegion property isn't well documented

It doesn't seem to very clear what visible region is or how it can be used.

It need the visible region to construct a coordinate system for an UTM Overlay. Works perfect, but I cannot figure out how the qvisibleRegion works exactly even with reading the sourcecode. IMHO visibleRegion should be (that is what I'm currently using):

Map {
  id: mapBase

  onCenterChanged: {
    var region = QtPositioning.path();
    region.addCoordinate(mapBase.toCoordinate(Qt.point(0, 0)));
    region.addCoordinate(mapBase.toCoordinate(Qt.point(width, 0)));
    region.addCoordinate(mapBase.toCoordinate(Qt.point(width, height)));
    region.addCoordinate(mapBase.toCoordinate(Qt.point(0, height)));

    utmOverlay.visibleRegion = region
  }
}

UTMOverlay {
  id: utmOverlay
}

The visibleRegion of Map also delivers a polygon but it is not clear how it correlates to the screen coordinates. This is very important if one wants to use visibleRegion for the construction of an overlay component/mapping coordinate transformation.