Abstract The Open Test sequence eXchange (OTX) Core, defined in ISO 13209-2, describes the basic structure underlying every OTX document. ISO 13209-3:2012 extends the Core by a set of additional features, using the extension mechanism rules described in ISO 13209-2. The extensions defined in ISO 13209-3:2012 comprise features which allow diagnostic communication to a vehicle's diagnostic interface, flashing, executing diagnostic jobs, controlling measurement equipment, internationalization, working with physical units, accessing the environment, communication via a human machine interface (HMI) and other utility extensions. ISO 13209-3:2012 defines the OTX extension requirements and data model specifications. The requirements are derived from the use cases described in ISO 13209-1. The data model specification aims at an exhaustive definition of all features of the OTX extensions which have been implemented to satisfy the requirements. ISO 13209-3:2012 establishes rules for the syntactical entities of each extension. Each of these syntactical entities is accompanied by semantic rules which determine how OTX documents containing extension features are to be interpreted. The syntax rules are provided by UML class diagrams and XML schemas, whereas the semantics are given by UML activity diagrams and prose definitions.