The left pane contains a 2 level hierarchy of categories and subcategories. As the user selects different subcategories the designer wants the right pane’s content to be updated with the corresponding information. Just like Outlook, the designer wants the subcategories to be displayed within expanding and collapsing panels. Accordion AjaxControlToolkit.
The Accordion is not the only toolkit control that allows you to provide multiple panes of content and display them one at a time, the Tab control does this as well. The trick is deciding what control better meets your requirements.