When an application creates a primary window, the system automatically adds a taskbar button for that window and removes it when that window closes. For some specialized types of applications that run in the background, a primary window may not be necessary. In such cases, make certain you provide reasonable support for controlling the application using the commands available on the application's icon; it should not appear as an entry on the taskbar, however. Similarly, the secondary windows of an application should also not appear as a taskbar button.
The taskbar window buttons support drag and drop, but not in the conventional way. When the user drags an object over a taskbar window button, the system automatically restores the window. The user can then drop the object in the window.