(String) If specified, and if the control has a clickable, then the specified string will appear automatically just above or below the control when the mouse hovers over it for about one second. This is useful for additional explanation about what will happen when the control is clicked.

If you want to associate a tooltip with a static text control, it must have the MBF_KBD style flag and a click string defined in the Alternate Dialog Coordinate System parameter. (Otherwise the control is not really "clickable", or more precisely, the click action is ignored and so is the tooltip feature.) If you want to have a tooltip even though there is no click action, the workaround is to use "VK_NULL" from the Virtual Key Symbolic Names table.