用于创建目录问题的magento示例

在Magento文档中，在创建目录类别api时，很少有参数设置为2的示例，但没有解释实际是什么！

2只是使用。

$result = $client->call($session, 'catalog_category.create', array(2, array( // <<-- what is this 2 ??
    'name' => 'Category name',
    'is_active' => 1,
    'position' => 1,
    //<!-- position parameter is deprecated, category anyway will be positioned in the end of list
    //and you can not set position directly, use catalog_category.move instead -->
    'available_sort_by' => 'position',
    'custom_design' => null,
    'custom_apply_to_products' => null,
    'custom_design_from' => null,
    'custom_design_to' => null,
    'custom_layout_update' => null,
    'default_sort_by' => 'position',
    'description' => 'Category description',
    'display_mode' => null,
    'is_anchor' => 0,
    'landing_page' => null,
    'meta_description' => 'Category meta description',
    'meta_keywords' => 'Category meta keywords',
    'meta_title' => 'Category meta title',
    'page_layout' => 'two_columns_left',
    'url_key' => 'url-key',
    'include_in_menu' => 1,
)));

有人知道它是什么吗？