These four widgets denote an empty place in the dialog. They differ in whether they are stretchable or not. Empty is not stretchable in either direction. It can be used in a `ReplacePoint, when currently no real widget should be displayed. HStretch and VStretch are stretchable horizontally or vertically resp., HVStretch is stretchable in both directions. You can use them to control the layout.
1.4.4