Source

File: wp-includes/class-wp-customize-nav-menus.php

public function customize_dynamic_partial_args( $partial_args, $partial_id ) {

		if ( preg_match( '/^nav_menu_instance\[[0-9a-f]{32}\]$/', $partial_id ) ) {
			if ( false === $partial_args ) {
				$partial_args = array();
			}
			$partial_args = array_merge(
				$partial_args,
				array(
					'type'                => 'nav_menu_instance',
					'render_callback'     => array( $this, 'render_nav_menu_partial' ),
					'container_inclusive' => true,
					'settings'            => array(), // Empty because the nav menu instance may relate to a menu or a location.
					'capability'          => 'edit_theme_options',
				)
			);
		}

		return $partial_args;
	}