[This is preliminary documentation and subject to change.]
Often, simple elements can generate events. Since these elements are not accessible objects and don't support Active Accessibility, they present a special problem for accessibility aids. Therefore, Active Accessibility places the responsibility of supporting the IAccessible interface on the element's parent object. With every event notification, the client application's WinEventProc hook procedure receives a child identifier in the idChild parameter, which specifies the simple element that generated the event. The accessible object that contains the element supports IAccessible on the child element's behalf. To access information about the child element, specify the child identifier in whatever method you call. If no child is associated with the event, idChild is CHILDID_SELF. For more information, see Object and Child Identifiers. All pertinent IAccessible methods accept the pvarChild parameter specifically for the purpose of finding child objects.