[This is preliminary documentation and subject to change.]
In the case of window objects only, both the parent and the child send this event. The child window sends the event when created, and the parent sends it after creation is complete. Note that the child window is responsible for sending this event before the parent.
If the client is using synchronous callbacks, it should not attempt to retrieve the parent object (by using IAccessible::get_accParent) until after control is yielded to the operating system.