Diagrams can be thought of as graphs with vertices and edges that are arranged according to a certain algorithm. 

Each diagram belongs to a certain diagram type (for example, UML 2.0 Class Diagram). A set of model elements available for use on a diagram depends on the diagram type. 

Diagrams exist within the context of a namespace (or a package). You have to create or open a project or project group before creating a new diagram. When the UML modeling support is activated, the project-level package diagram is created by default. You can create the various UML diagrams in the project.  

In addition to the standard properties of diagrams and their elements, you can create user properties, represented by the Name-Value pair.