In any Core screen, if you select a few items in the list view and then choose to Export as CSV or View Reports from the More menu, Core does not restrict the exported data (or reports) to the selected items. It includes all items from the list. If you want to restrict it to a few items, you can use the screen filters. Check the Core Help for more on that. After filtering a screen and using the Export as CSV or View Reports option, Core restricts the data to the items visible in the filtered view.