The Click event is fired when the user releases the left mouse button over the control. Use a MouseDown or MouseUp event procedure to specify actions that will occur when a mouse button is pressed or released. Unlike the Click and DblClick events, MouseDown and MouseUp events lets you distinguish between the left, right, and middle mouse buttons. You can also write code for mouse-keyboard combinations that use the SHIFT, CTRL, and ALT keyboard modifiers. The control fires the SelectItem event when user clicks an item. The SelectGroup event is fired when the user clicks the group's header. Use the ExpandOnClick property to specify whether the user can expand or collapse the groups by clicking the caption of the group.