Source

File: wp-includes/class-walker-comment.php

public function start_lvl( &$output, $depth = 0, $args = array() ) {
		$GLOBALS['comment_depth'] = $depth + 1;

		switch ( $args['style'] ) {
			case 'div':
				break;
			case 'ol':
				$output .= '<ol class="children">' . "\n";
				break;
			case 'ul':
			default:
				$output .= '<ul class="children">' . "\n";
				break;
		}
	}