symfony creating custom form types

// src/Form/Type/OrderType.php
namespace App\Form\Type;

use App\Form\Type\ShippingType;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class OrderType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            // ...
            ->add('shipping', ShippingType::class)
        ;
    }

    // ...
}

// src/Form/Type/ShippingType.php
namespace App\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\OptionsResolver\OptionsResolver;

class ShippingType extends AbstractType
{
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'choices' => [
                'Standard Shipping' => 'standard',
                'Expedited Shipping' => 'expedited',
                'Priority Shipping' => 'priority',
            ],
        ]);
    }

    public function getParent()
    {
        return ChoiceType::class;
    }
}