what is UI Virtualization

If we have a scrolling ListBox that needs to be data bound to million items.
But we know only 50 are visible at any time.
Without UI virtualization, the ListBox would create a million ListBoxItems even though only a hundred of them are visible.
With UI virtualization, ListBox would only create/show 50 ListBoxItems (will rerender rest while scrolling.).

btw ... this is all about UI virtualization. Data is loaded already with million rows. But Items control did not loaded million items.

'data virtualization' is one step deeper. Instead, it only loads the ones it needs to display.