There are two types of class diagrams used in Modeling:

• Package (namespace) diagrams. These diagrams are referred to as package diagrams in design projects, and namespace diagrams in implementation projects. They are stored as XML files in the ModelSupport_%PROJECTNAME% folder of the project group with the file extension .txvpck.
• Logical class diagrams. These diagrams are stored as XML files in the ModelSupport_%PROJECTNAME% folder of the project group with the file extension .txvcls.

Modeling automatically creates a default namespace diagram for the project and for each subdirectory under the project directory. The default project diagram is named default; the default namespace (package) diagrams are named after the respective namespaces (packages).  

You create logical class diagrams manually by using the AddClass Diagram or AddOther Diagram command on the project context menu.