A diagram element, also called a shape, is a graphical or textual element that represents a model element in a diagram.
Diagram elements textually or graphically represent their respective model elements in the diagram editor. In diagrams, diagram elements illustrate conceptually related model elements. Diagram elements show all or part of the semantics of their respective model elements; however, diagram elements do not add semantic information to the model itself.
You can change the appearance and location of a diagram element. If you modify the properties of a diagram element, you change the value of both the selected diagram element and its underlying model elements. For example, if you rename the Name property, you change the property value of the model element and all of the diagram elements that represent it regardless of whether you change the name in the Model Explorer view, the Properties view, or in the diagram editor.
Using diagram elements, you can develop different types of UML and non-UML diagrams. In the product, there are two types of diagram elements: UML diagram elements and geometric shapes, which are non-UML diagram elements. UML diagram elements represent UML model elements in diagrams and have well-defined semantics with guidelines on how to use these UML diagram elements and what they represent. Geometric shapes (such as ovals, diamonds, and rectangles) do not have any UML semantics and do not have to adhere to UML standards; you can define guidelines on how you use them. Both types of diagram elements can appear in the same diagram; however, if you validate your diagram for UML compliance, the software validates only the model elements that the UML diagram elements represent.
You can use UML diagram elements and geometric shapes in your models to represent different elements. For example, you can use UML diagram elements such as activities, initial nodes, final nodes, control flows, swimlanes, and decisions to model a workflow. You can use geometric shapes (such as a rectangle shape) in a use case diagram to separate the internal use cases from the external actors.