I have a class with several attributes. Among these there is an optional one, applicable only under certain conditions. To avoid the presence of many null attributes, you could detach the optional attribute in a class. In this case, the class with only the optional attribute is a weak class or a normal class? Obviously, detaching the attribute from the class will not mark the attribute as optional but the multiplicity that connects the new class with the old one. multiplicity (1,1) and (0,1)