Custom commands can be of two types:
JavaScript commands are defined in the web layout as commands of type Invoke Script. They are used primarily to interact with the Viewer, and can use the Viewer API.
Web Server Extension pages can be added to the web layout in two different ways. In one method, the web layout includes a home page. This home page is loaded in the task pane when the map first displays, and can be re-loaded by clicking the Home icon in the task bar. The home page can load other pages as needed.
In addition, other task pages can be defined in the web layout as commands of type Invoke URL. These commands can be added to the tool bar, task list, or context menu. When a user selects one of these commands the corresponding URL is often loaded into the task pane, though it can also be loaded into a hidden frame so it is not visible.
Because Web Server Extension pages are created at the web tier before being passed to the Viewer, they can use both the Web Server Extension API and the Viewer API.