Hierarchical Select

In this example, the dropbox is enabled, and a title and limit are set for it. Also, the selection will not be reset after you add an item to the dropbox.
Try adding a term and you will get a warning, stating that you cannot select any additional items.
Show config

Array
(
    [module] => hs_taxonomy
    [params] => Array
        (
            [vid] => 2
            [exclude_tid] => 
            [root_term] => 
            [entity_count_for_node_type] => 
        )

    [save_lineage] => 0
    [enforce_deepest] => 1
    [entity_count] => 0
    [require_entity] => 0
    [level_labels] => Array
        (
            [status] => 0
        )

    [dropbox] => Array
        (
            [status] => 1
            [title] => Car park
            [limit] => 3
            [reset_hs] => 0
        )

    [editability] => Array
        (
            [status] => 0
        )

    [resizable] => 1
    [render_flat_select] => 0
)

In this example, the dropbox still is enabled, but you are no longer forced to select an item from the deepest level. Level labels have been entered. And the entire lineage is being saved. Finally, the ability to create new items is enabled.
Try adding Paris, you will see that France will be selected as well.
Also try creating some new terms, Hierarchical Select makes this trivially easy and painless.
Show config

Array
(
    [module] => hs_taxonomy
    [params] => Array
        (
            [vid] => 6
            [exclude_tid] => 
            [root_term] => 
            [entity_count_for_node_type] => 
        )

    [save_lineage] => 1
    [enforce_deepest] => 0
    [entity_count] => 0
    [require_entity] => 0
    [level_labels] => Array
        (
            [status] => 1
            [labels] => Array
                (
                    [0] => Country
                    [1] => City
                )

        )

    [dropbox] => Array
        (
            [status] => 1
            [title] => Places
            [limit] => 0
            [reset_hs] => 1
        )

    [editability] => Array
        (
            [status] => 1
            [item_types] => Array
                (
                    [0] => country
                )

            [1] => city
            [allowed_levels] => Array
                (
                )

            [allow_new_levels] => 1
            [max_levels] => 2
        )

    [resizable] => 1
    [render_flat_select] => 0
)