code: private function readBstoreContainer()
      {
          $length = Xls::getInt4d($this->data, $this->pos + 4);
          $recordData = substr($this->data, $this->pos + 8, $length);
  
          // move stream pointer to next record
          $this->pos += 8 + $length;
  
          // record is a container, read contents
          $bstoreContainer = new BstoreContainer();
          $this->object->setBstoreContainer($bstoreContainer);
          $reader = new self($bstoreContainer);
          $reader->load($recordData);
      }
docs: Read BstoreContainer record (Blip Store Container).