This property is a string that contains the id of the PageView control that should display the image displayed in this thumbnail view when you change the value of the selected property from false to true.

It is common for applications of a ThumbnailView control to display the selected thumbnail in a larger viewer and this property makes it easier for you to implement that kind of application. Simply set this property (for all of your ThumbnailView controls) to the id of the PageView you want to use and then add JavaScript to set the selected property of a ThumbnailView control to true when a user clicks on it. The control will automatically change the selected property back to false if you open a different image in the PageView.

You will typically want to adjust your HTML so that the selected ThumbnailView control is visibly different from the rest. The best way to do this is to add a propertyChanged event handler to each of your ThumbnailView controls. In that event handler, ignore all events except for changes to the selected property and either add or remove a CSS class based on the new value of that property.

Reference

ThumbnailView class
ThumbnailView Members