The Document Selector is a tool that helps to navigate through documents at runtime. An end-user can invoke it by pressing Ctrl+Tab or the Ctrl+Shift+Tab keys. By default, all Documents within a View are displayed within a Document Selector. To manually set which of these should be displayed, use the IDocumentDefaultProperties.ShowInDocumentSelector property for individual Documents or the ShowInDocumentSelector property for all a View's Documents.
See the Document Selector topic to learn more.