// demoing by sticking all the available values onto node_content
$items['content_type'] = array(
    'title' => t('Node content'),
    'weight' => -10,
    'single' => TRUE, // only provides a single content type
    'content_types' => 'panels_admin_content_types_node_content',
    'render callback' => 'panels_content_node_content',
    'add callback' => 'panels_admin_edit_node_content',
    'edit callback' => 'panels_admin_edit_node_content',
    'title callback' => 'panels_admin_title_node_content', // all the new ones follow
    'visibility control' => 'panels_admin_visibility_control_node_content',
    'visibility submit' => 'panels_admin_visibility_submit_node_content',
    'visibility render' => 'panels_content_visibility_node_content',
    'visibility serialize' => TRUE,
    'ignore roles' => FALSE,
    'roles and visibility' => TRUE,
    'form control' => 'panels_admin_form_control_node_content',
);