Source

File: wp-includes/class-wp-rewrite.php

public function add_rule( $regex, $query, $after = 'bottom' ) {
		if ( is_array( $query ) ) {
			$external = false;
			$query    = add_query_arg( $query, 'index.php' );
		} else {
			$index = false === strpos( $query, '?' ) ? strlen( $query ) : strpos( $query, '?' );
			$front = substr( $query, 0, $index );

			$external = $front != $this->index;
		}

		// "external" = it doesn't correspond to index.php.
		if ( $external ) {
			$this->add_external_rule( $regex, $query );
		} else {
			if ( 'bottom' === $after ) {
				$this->extra_rules = array_merge( $this->extra_rules, array( $regex => $query ) );
			} else {
				$this->extra_rules_top = array_merge( $this->extra_rules_top, array( $regex => $query ) );
			}
		}
	}