[This is preliminary documentation and subject to change.]
Server applications send this notification when the item being selected is within the same container as the last selected object.
The hwnd and idObject parameters of the WinEventProc callback function describe the container, and the idChild parameter identifies the object that is selected. The idChild parameter can be OBJID_WINDOW if the selected child is a window that also contains objects.
Do not confuse object selection with object focus. For more information, see Accessible Object Selection and Focus.