[Ed B.]

This is about 5.3, and I don't know about in other versions. A quick look at the php code shows that there is an array defined in classes/XLite/View/FormModel/Product/Info.php called

$schema

So if you want to make required, for example, images, then you would want to set

 $schema[self::SECTION_DEFAULT]['images']['required'] = true;
            $schema[self::SECTION_DEFAULT]['images']['constraints']['Symfony\Compon
ent\Validator\Constraints\NotBlank'] = [
                'message' => static::t('This field is required'),
            ];

Now, unfortunately I don't know if there is a method defiend somewhere for changing this array.