When you click on Preliminary selection under the Window functions, the right portion of the window will open and the left one will no longer be active.
In the right portion of the window, you will see all orders (within the interval that you selected in the left portion of the window) that you can print transport labels for. You can deselect the orders that you do not want to print transport labels for. To return to the left portion, just click on the Cancel button.
When the right portion of the window is activated, you can also select Preview under the Window functions, in order to preview the transport label of the row you have selected on the screen.