Message for a window in which the destination is another window

I would need an explanation about the destination of the messages.
The WM_MESSAGE structure has the hWin field described as "Destination window":

struct WM_MESSAGE {
  int MsgId;        	/* type of message */
  WM_HWIN hWin;     	/* Destination window */
  WM_HWIN hWinSrc;  	/* Source window  */
  union {
	const void * p;        	/* Some messages need more info ... Pointer is declared "const" because some systems (M16C) have 4 byte const, byte 2 byte default ptrs */
	int v;
	GUI_COLOR Color;
  } Data;
};

Also, the functions for sending messages (e. g. WM_SendMessage) have the hWin parameter used to indicate the target window (or at least that's what I understood).

Could you give me an example in which the hWin field of the structure and the hWin parameter of the functions are populated differently? When can such a case be useful or indispensable?

best regards
Max