code: static public function calculateDeliveryRemainingQuantity($item)
      {
          if ($item instanceof SupplierOrderItemInterface) {
              return $item->getQuantity() - static::calculateReceivedQuantity($item);
          }
  
          if (!$item instanceof SupplierDeliveryItemInterface) {
              throw new InvalidArgumentException(
                  "Expected instance of " .
                  SupplierOrderItemInterface::class . " or " .
                  SupplierDeliveryItemInterface::class
              );
          }
  
          $orderItem = $item->getOrderItem();
  
          $result = $orderItem->getQuantity() - static::calculateReceivedQuantity($orderItem);
  
          if (0 < $item->getQuantity()) {
              $result += $item->getQuantity();
          }
  
          return $result;
      }
docs: Calculate the given supplier order item's delivery remaining quantity.
  
  @param SupplierOrderItemInterface|SupplierDeliveryItemInterface $item
  
  @return float