This tab is available only for Portal engine templates for users with sufficient permissions. Here you can add (), remove (
) and configure (
) web parts and widgets, manage their zones and edit the HTML layout of the current document (using the Edit layout (
) button in the top right corner of the page template).
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?cms_content_design.htm