The KML window shows the contents of a single KML or KMZ dataset that you either drag and drop onto the EarthBrowser window, or from a link that you clicked on while in the Web Browser. It is a simple hierarchical list that looks and behaves just like the dataset list the Controls Window. You can drag and drop items from the KML window into your own personal data to store them. However since this is a temporary dataset, once you close the window all of the contents are removed from the earth.
Example KML window