Source

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

public function start_dynamic_sidebar( $index ) {
		array_unshift( $this->current_dynamic_sidebar_id_stack, $index );
		if ( ! isset( $this->sidebar_instance_count[ $index ] ) ) {
			$this->sidebar_instance_count[ $index ] = 0;
		}
		$this->sidebar_instance_count[ $index ] += 1;
		if ( ! $this->manager->selective_refresh->is_render_partials_request() ) {
			printf( "\n<!--dynamic_sidebar_before:%s:%d-->\n", esc_html( $index ), intval( $this->sidebar_instance_count[ $index ] ) );
		}
	}