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MSDN

WPARAM type changes, in Win32

Changing Message Handles

wParam

The low-order word of wParam identifies the command ID of the menu item, control, or accelerator. The high-order word of wParam specifies the notification message if the message is from a control. If the message is from an accelerator, the high-order word is 1. If the message is from a menu, the high-order word is 0.

lParam

Identifies the control that sends the message if the message is from a control. Otherwise, lParam is 0.

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