The Selectors allow us to apply a different DataTemplate or Style on an item, based on some custom logic. They are used whenever you need to visualize the same data in different way, depending on one or more conditions.
For example you have a several tabs, which contain data about different animals or plants. You want to display the animals in tabs with special header template (with a yellow background) and the plants in tabs with a header template with a green background.
Next, create a class file and name it HeaderSelector. The HeaderSelector class must derive from the DataTemplateSelector class, which is located into the Telerik.Windows.Controls assembly.
Please note that RadTabControl exposes an ItemTemplateSelector that applies a DataTemplateSelector as an HeaderTemplateSelector to all RadTabItems.