推荐：http://weblogs.asp.net/soever/archive/2007/05/03/sharepoint-features-elements-scope-and-other-info.aspx
Location and Group ID
To define a custom action for a particular menu, you must identify the menu by setting the location to the appropriate Windows SharePoint Services namespace, and by using the ID that Windows SharePoint Services uses to identify the specific location.
For example, to add a custom action to the Site Settings page, set the Location attribute of the CustomAction element to Microsoft.SharePoint.SiteSettings.and specify a particular area within the page through the GroupId attribute.
Different actions may require using different CustomAction attributes to identify the menu in which to place a custom menu item. But you may also need to specify other parameters for the action, for example, to specify a version, user permissions required to perform the action, or placement in relation to existing actions in the menu. The custom actions of the following example show a variety of attributes.
See Default Custom Action Locations and IDs for a list of the default custom action IDs and locations that are used in an installation of Windows SharePoint Services.
