IsContentPreserved property provides ContentPresenter for every TabItem content (more precisely, every previously selected item). Thus when you select particular item its content will be inserted into the visual tree via dedicated content presenter. When next item is selected, its content will be inserted dynamically into the visual tree again via dedicated content presenter and the previously visible content will be collapsed.
So if all tab items have been selected, their content will be part of the visual tree. If item has not yet been selected its content won't be in the visual tree.
Hope this helps.
the Telerik team
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