You can create extensions to the user interface layer of the vSphere Client that extend the GUI with components specific for your vSphere environment.

The vSphere Client is a Web browser-based application that provides extensible plug-in architecture. The user interface layer contains every visual component of the application, including data views, portlets, and navigation controls.

You can add new features or elements to the HTML application by creating user interface extensions. An HTML user interface plug-in contains one or more extensions, which add HTML GUI elements to the vSphere Client user interface.