Model.md

class User extends \atk4\data\Model
{
    function init() {
        parent::init();

        $this->addField('name');
        $this->addField('email');
    }
}

UI

$form = $app->add(['Form', 'segment']);
$form->setModel(new User($db), ['email', 'password']);

$form->addField('is_accept_terms', null, ['type'=>'boolean']);
$form->submitButton->set('Register');

$form->onSubmit(function ($form) {
    if (!$form->model['is_accept_terms']) {
        return $form->error('is_accept_terms', 'Please accept terms');
    }

    $form->model->save();

    return $form->success('Thank you');
});