The WM_RENDERFORMAT message is sent to the clipboard owner if it has delayed rendering a specific clipboard format and if an application has requested data in that format. The clipboard owner must render data in the specified format and place it on the clipboard by calling the SetClipboardData function.
WM_RENDERFORMAT uFormat = (UINT) wParam; // clipboard format
If an application processes this message, it should return zero.
When responding to a WM_RENDERFORMAT message, the clipboard owner must not open the clipboard before calling SetClipboardData.