The Telerik RadAutoCompleteBox uses an Entries collection to represent the items a user has selected. Understanding how Entries works is essential when binding data, loading saved values, or reacting to user selections during postbacks or AJAX requests.
Personally, I've only ever used it in 'single' entry mode, but it is pretty versatile and allows a lot more than that.
If you're not familiar with the RadAutoCompleteBox, go to the menu and read that article first.