Source

File: wp-includes/class-wp-customize-manager.php

public function add_section( $id, $args = array() ) {
		if ( $id instanceof WP_Customize_Section ) {
			$section = $id;
		} else {
			$section = new WP_Customize_Section( $this, $id, $args );
		}

		$this->sections[ $section->id ] = $section;
		return $section;
	}