In composite structure diagrams, a part is a diagram element that represents a set of one or more instances that a containing structured classifier owns. A part describes the role of an instance in a classifier. You can create parts in the structure compartment of a classifier, and in several UML diagrams such as composite structure, class, object, component, deployment, and package diagrams.
Parts are owned by composition and the composite structure diagram models how the parts are connected in the containing classifier. Each part is a specific usage of a type. The type defines the kinds of objects that can be bound, or referenced, to a role. You can have multiple parts with the same type and each part can have a different set of relationships to other parts. You can join parts with connectors to specify the communication between them.
As the following figure illustrates, a part is displayed in the diagram frame as a solid rectangle that contains the name of the part, the type, and any specified multiplicity.
The example in the figure shows a containing classifier named Car. The composite structure diagram shows the four composite parts of the car, which each represent a wheel type. The frontaxle connector links the front wheels and the rearaxle connector links the rear wheels.