You can control the way panel items expand via the ExpandMode property of RadPanelBar. The supported modes are:

• MultipleExpandedItems: More than one item can be expanded at a time.
• SingleExpandedItem: Only one item can be expanded at a time. Expanding an item collapses the previously expanded one.
• FullExpandedItem: Only one item can be expanded at a time. The expanded area occupies the entire height of the RadPanelbar