With the Select Card window you can select a card, either for the Card View, or for parameters that need a card as an argument, like the target card for the Go to Card by Name standard command for command controls.
With a combo box for selecting the background and a list that shows all currently existing cards for the selected background the window so far offers only a very basic functionality, and it lags behind the Select Card dialog that MoStacks offers on UIQ3.
Double-clicking an entry of the card list has the same result as clicking the Select button.