The StyleVisionControlPlaceHolder control is required to host an application helper window, see also Integration at Document Level. For example, in the code listing below, the HTML object tag is used to instantiate a control that will host the Message window.
<!-- create Message window placeholder control. -->
Notice that the PlaceholderWindowID parameter is set to -1, which means that, initially, no helper window is shown. Whenever a file is open, the method DisplayHelperWindowsContent() is invoked. This method assigns the appropriate PlaceholderWindowID to each control. For the list of possible values of PlaceholderWindowID, see StyleVisionControlPlaceholderWindow.