code: protected function getTransmissionId(QtiRunnerServiceContext $context, $itemId = null)
      {
          if (is_null($itemId)) {
              $itemId = $context->getCurrentAssessmentItemRef();
          } elseif ($itemId != $context->getCurrentAssessmentItemRef()) {
              throw new \common_exception_NotImplemented('Item variables can be stored only for the current item');
          }
  
          $sessionId = $context->getTestSession()->getSessionId();
          $currentOccurrence = $context->getTestSession()->getCurrentAssessmentItemRefOccurence();
  
          return $sessionId . '.' . $itemId . '.' . $currentOccurrence;
      }
docs: Build the transmission based on context and item ref id to store Item variables
  
  @param QtiRunnerServiceContext $context
  @param null $itemId The item ref identifier
  @return string The transmission id to store item variables
  @throws \common_exception_NotImplemented If the given $itemId is not the current assessment item ref