Check buttons are used to allow the user to select any number of options from a fixed list of possible options. They are most usefull when you have a small number of options and the user can select any combination of them. When a check button is clicked, it's state is toggled from checked to unchecked or unchecked to checked.
If you have registered the onCommand callback, it will be called with the id of the check box as the id parameter and $$BN_CLICKED as the code.
You can get or set the check state of a check button with the WinXButton_SetCheck and WinXButton_GetCheck functions.