UML Class Diagram – How to Represent Multiple Choice Attribute

An association links one class to another to indicate a relation between them. For example, you have an association between Customer and Order which (with no further information) indicates that:

1. A customer can have an order or maybe several orders,

2. An order can belong to a customer or maybe shared by several customers.

The diagram in your question gives an additional info: there is one and only one customer by order, and zero or more orders by customer, so the rules become:

1. A customer have zero or more orders,

2. An order belongs to one and one only customer.

While aggregation is used in the association between Order and OrderDetail, it's not used in the association between Customer and Order. This is done to indicate the life cycle dependency. If an instance of the Order is destroyed, all instances of OrderDetail inside it would be destroyed too; on the other hand, the instances of Order are independent of the life cycle of the Customer instance, thus no aggregation here.

This doesn't mean that an order can exist without a customer: the 1 ↔ 0..* association explicitly forbids the orders which have no customers. The difference between a 1 ↔ 0..* association and the aggregation really happens on life cycle level.

You could use Extend in this case.

Example include and extend: UC “login” includes UC “sign up”: The login page can be accesses straight away, but if you haven’t signed up the alt path would lead you to the sign up page . You must complete this UC to get through. You can assess the sing up page directly as well. So for reuse you could make this two use cases, instead of an alt path and include the sing up UC.

UC “edit profile” extends UC “login”: The UC “Login” always has a pop-up when you login to ask if you want to change your profile. You don’t have to do this to accesses the site. You can accesses the profile edit page from several places, with its own UC of course. You would draw this relationship as an extend because its optional to get through.