code: private function writeExternalsheetBiff8()
      {
          $totalReferences = count($this->parser->references);
          $record = 0x0017; // Record identifier
          $length = 2 + 6 * $totalReferences; // Number of bytes to follow
  
          $supbook_index = 0; // FIXME: only using internal SUPBOOK record
          $header = pack('vv', $record, $length);
          $data = pack('v', $totalReferences);
          for ($i = 0; $i < $totalReferences; ++$i) {
              $data .= $this->parser->references[$i];
          }
  
          return $this->writeData($header . $data);
      }
docs: Writes the Excel BIFF EXTERNSHEET record. These references are used by
  formulas.