Item Template contains the content appears in the panel item immediately below its text. The item cannot be collapsed nor expanded. If the item does not have any text, then the contents of the Item Template become the item itself. Content Template contains the content renders as a sub-group of the target panel item. The item can be expanded and collapsed as if it had child panel items. If the item does not have text, the contents of the Content Template become the item itself and cannot be expanded/collapsed.
RadPanelBar has items with content templates you can access the controls embedded in the content templates directly by ID, regardless of the level of the template itself.RadPanelBar has items with item templates that have embedded controls, you may want to access the properties of those embedded controls. To get a reference to the embedded control, locate the RadPanelBarItem that has the item template, and use its FindControl method.
Please take a look into the following documentations fore more information.
Accessing Controls Inside Templates
Hope this helps.