The Items tab summarizes the items to be exported in your model. Only the items that are in the project and that the selected export format supports will be visible. The number of associated items to be exported is shown in brackets next to the item type. The example above shows that 1 Line, 15 Curves, and 1 Surface will be exported and based on the format selected, none of the Points item types can be exported (disabled).
You can use the "Check All" and "Uncheck All" buttons at the bottom of this tab to quickly select or deselect all items.