CTLOP_EBCH (opcode 11) is only used with Tab controls, in order to set (change) the current pane. Each tab in a Tab control is normally defined to send a different exitcode when clicked. Upon receiving the exitcode, the program needs to clear the controls from the current pane, activate the clicked pane, and then recreate the controls that belong on that pane. See the section Tab Control below for a more complete example.