Marking of items is currently implemented through so called "clipboard" functionality.

The clipboard is a temporary (session-level) personal collection of items. At the current time, you can create only one clipboard collection at the time. If you need more, or if you need permanent collections, see Collections instead.

Click the (+) icon on the listing, close to item, to put the item on the clipboard (you can't use CTRL+C in this case).

To remove the item from the clipboard, click the (-) icon in the item row on the clipboard pane.

See also

Clipboard navigator