MigrateDestMgr.php

  /**
   * {@inheritdoc}
   */
  public function processDefinition(&$definition, $plugin_id) {
    parent::processDefinition($definition, $plugin_id);

    // Loop over each of the definitions and categorize it as config or content.
    if (!empty($definition['class'])) {
      $configuration_classes = [
        EntityConfigBase::class,
        Config::class,
        UserData::class,
        ComponentEntityDisplayBase::class,
      ];
      // Do not overwrite an otherwise defined category.
      if (empty($definition['category'])) {
        if ($this->classInArray($definition['class'], $configuration_classes)) {
          $definition['category'] = MigrateDestination::CATEGORY_CONFIGURATION;
        }
        else {
          $definition['category'] = MigrateDestination::CATEGORY_CONTENT;
        }
      }
    }
  }

MigratePluginMgr.php

  public function processDefinition(&$definition, $plugin_id) {
    parent::processDefinition($definition, $plugin_id);

    // Loop over each of the definitions and categorize it as config or content.
    $destination = $this->migrateDestinationPluginManager->getDefinition($definition['destination']['plugin'], FALSE);
    if (!empty($destination['class'])) {
      $configuration_classes = [
        EntityConfigBase::class,
        Config::class,
        UserData::class,
        ComponentEntityDisplayBase::class,
      ];
      // Do not overwrite an otherwise defined category.
      if (empty($definition['category'])) {
        if (!empty($destination['category'])) {
          $definition['category'] = $destination['category'];
        }
        elseif ($this->classInArray($destination['class'], $configuration_classes)) {
          $definition['category'] = MigrateDestination::CATEGORY_CONFIGURATION;
        }
        else {
          $definition['category'] = MigrateDestination::CATEGORY_CONTENT;
        }
      }
    }
  }