code: private function writeGuts()
      {
          $record = 0x0080; // Record identifier
          $length = 0x0008; // Bytes to follow
  
          $dxRwGut = 0x0000; // Size of row gutter
          $dxColGut = 0x0000; // Size of col gutter
  
          // determine maximum row outline level
          $maxRowOutlineLevel = 0;
          foreach ($this->phpSheet->getRowDimensions() as $rowDimension) {
              $maxRowOutlineLevel = max($maxRowOutlineLevel, $rowDimension->getOutlineLevel());
          }
  
          $col_level = 0;
  
          // Calculate the maximum column outline level. The equivalent calculation
          // for the row outline level is carried out in writeRow().
          $colcount = count($this->columnInfo);
          for ($i = 0; $i < $colcount; ++$i) {
              $col_level = max($this->columnInfo[$i][5], $col_level);
          }
  
          // Set the limits for the outline levels (0 <= x <= 7).
          $col_level = max(0, min($col_level, 7));
  
          // The displayed level is one greater than the max outline levels
          if ($maxRowOutlineLevel) {
              ++$maxRowOutlineLevel;
          }
          if ($col_level) {
              ++$col_level;
          }
  
          $header = pack('vv', $record, $length);
          $data = pack('vvvv', $dxRwGut, $dxColGut, $maxRowOutlineLevel, $col_level);
  
          $this->append($header . $data);
      }
docs: Write the GUTS BIFF record. This is used to configure the gutter margins
  where Excel outline symbols are displayed. The visibility of the gutters is
  controlled by a flag in WSBOOL.
  
  @see writeWsbool()