When the user clicks mouse button 2 on an item within the shell's namespace (that is, file, directory, server, work group, and so on), it creates the default context menu for the type of item and then loads context menu extensions that are registered for the type (and its base type) so that they can add extra menu items. The context menu extensions are registered at the following location.
HKCR\{ProgID}\shellex\ContextMenuHandlers