Thanks for the answer Lyle.

But the point is that, in the options of the attribute there are radio buttons that force you to select one of the attributes as predefined, so the result is that the attribute is always mandatory.

If you mark it as required is mandatory because the drupal api form forces you to select one (and then the predefined radio buttons are not used)
If you dont mark it as required, is mandatory too, because one of the attributes is selected by default and you cannot left it blank.