It's exactly the same as for the table. It is a custom component, the LovBoxRenderer, that is displaying the information corresponding to a country oid and you can select what field should be displayed in the combo box. As usual, no server call is made since the LOV is cached on the client.
Note that if you go on the combo box, the tooltip will display the internal format of the LOV (first the oid of the country, then the fields...), ie. the LovValue corresponding to the selected country.