A tooltip control can support any number of tools. To support a particular tool, you must register the tool with the tooltip control by sending the TTM_ADDTOOL message to the tooltip control. The message includes the address of a TOOLINFO structure, which provides information the tooltip control needs to display text for the tool. The cbSize member is required and must specify the size of the structure.
A tooltip control supports tools implemented as windows (such as child windows or control windows) and as rectangular areas within a window's client area. When you add a tool implemented as a rectangular area, the hwnd member of TOOLINFO must specify the handle of the window that contains the area, and the rect member must specify the client coordinates of the area's bounding rectangle. In addition, the uId member must specify the application-defined identifier for the tool.
When you add a tool implemented as a window, the uId member of TOOLINFO must contain the window handle of the tool. Also, the uFlags member must specify the TTF_IDISHWND value, which tells the tooltip control to interpret the uId member as a window handle.