The AUTOSAR Type Definiton Diagram (ardd) is used to define data and interface types. This is similar to create classes before being able to craete instances (objects). Use this diagram type when you must define interface and data types!
Example
When you open an AUTOSAR Type Definition Diagram the AUTOSAR Data Modeling Toolbox is automatically opened. This toolbox contains all neccessary elements and connectors to define AUTOSAR data and interface types.