речь шла о противоречии в твоих словах

из этого следует, что оно должно быть описано

Назначение параметра зависит от значения Message, и определяется разработчиком операционной системы

SendMessage Function

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The SendMessage function sends the specified message to a window or windows. It calls the window procedure for the specified window and does not return until the window procedure has processed the message.

To send a message and return immediately, use the !SendMessageCallback! or !SendNotifyMessage! function. To post a message to a thread's message queue and return immediately, use the !PostMessage! or !PostThreadMessage! function.

Syntax
LRESULT SendMessage( HWND hWnd,
UINT Msg,
WPARAM wParam,
LPARAM lParam
);

Parameters
hWnd
[in] Handle to the window whose window procedure will receive the message. If this parameter is HWND_BROADCAST, the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
Msg
[in] Specifies the message to be sent.
wParam
[in] Specifies additional message-specific information.
lParam
[in] Specifies additional message-specific information.

Return Value
The return value specifies the result of the message processing; it depends on the message sent.

Remarks
Applications that need to communicate using HWND_BROADCAST should use the !RegisterWindowMessage! function to obtain a unique message for inter-application communication.
The system only does marshalling for system messages (those in the range 0 to !WM_USER!). To send other messages (those above !WM_USER!) to another process, you must do custom marshalling.
If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine. If the specified window was created by a different thread, the system switches to that thread and calls the appropriate window procedure. Messages sent between threads are processed only when the receiving thread executes message retrieval code. The sending thread is blocked until the receiving thread processes the message. However, the sending thread will process incoming nonqueued messages while waiting for its message to be processed. To prevent this, use !SendMessageTimeout! with SMTO_BLOCK set. For more information on nonqueued messages, see !Nonqueued_Messages!.

Windows 95/98/Me: SendMessageW is supported by the Microsoft Layer for Unicode (MSLU). To use this, you must add certain files to your application, as outlined in !Microsoft_Layer_for_Unicode_on_Windows_95/98/Me_Systems!.

Example
For an example, see !Processing_Keyboard_Input!.

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Это ли не есть правильно?