Source

File: wp-admin/includes/post.php

function postbox_classes( $box_id, $screen_id ) {
	if ( isset( $_GET['edit'] ) && $_GET['edit'] == $box_id ) {
		$classes = array( '' );
	} elseif ( get_user_option( 'closedpostboxes_' . $screen_id ) ) {
		$closed = get_user_option( 'closedpostboxes_' . $screen_id );
		if ( ! is_array( $closed ) ) {
			$classes = array( '' );
		} else {
			$classes = in_array( $box_id, $closed, true ) ? array( 'closed' ) : array( '' );
		}
	} else {
		$classes = array( '' );
	}

	/**
	 * Filters the postbox classes for a specific screen and box ID combo.
	 *
	 * The dynamic portions of the hook name, `$screen_id` and `$box_id`, refer to
	 * the screen ID and meta box ID, respectively.
	 *
	 * @since 3.2.0
	 *
	 * @param string[] $classes An array of postbox classes.
	 */
	$classes = apply_filters( "postbox_classes_{$screen_id}_{$box_id}", $classes );
	return implode( ' ', $classes );
}