Form type:

use Symfony\Component\Form\AbstractType;

class ModifySettingsForm extends AbstractType {

    public function getBlockPrefix() {
        return 'modifySettings';
    }

}

Now calling

1. $this->get('form.factory')->createNamed('modifySettings', ModifySettingsForm::class)
2. $this->createForm(ModifySettingsForm::class)

in a controller leads to different results when rendering the template. While in the 1st case the form name actually is modifySettings, it is modify_settings_form in the 2nd case, so the block prefix is ignored and the name is generated using the class name.