[This is preliminary documentation and subject to change.]
The parameters received in the WinEventProc callback function describe the object that is receiving the keyboard focus.
There is only one focused child item in a parent. The focused child is the object that receives keystrokes at a given moment. Hence, server applications must only send this notification describing where the new focus is going. That is, a new item that gets the focus implies that the old one is losing it.
Do not confuse object focus with object selection. For more information, see Accessible Object Selection and Focus.