To understand how to add elements to a UML model, you first need to understand the distinction between a diagram and a model.
Models appear in a hierarchical tree view in the Model Explorer window.
Diagrams are views of a model that are represented by icons in the tree view, and by drawing pages in your UML model diagram drawing file.
Keep the following information in mind when you use the UML Model Diagram template:
You can add elements to a diagram by dragging icons from the tree view or shapes from stencils onto the drawing page.
When you drag a shape from a stencil onto the drawing page, the shape becomes part of the model. Many of the shapes you drag onto a drawing page, such as Class shapes, Use Case shapes, and Component shapes, are also automatically added to the tree view as icons.
Note: When you drag an icon from the tree view to the drawing page, you are creating a view of the model element (not a new model element).
When you add an element to the tree view, that element does not appear in a diagram unless you drag the icon representing the element from the tree view onto a drawing page.
You can duplicate elements in the tree view and you can drag element icons into different packages.
You can add different views of the same element to different diagrams by dragging the same element icon from the tree view onto different drawing pages.