This part of ISO 13209 defines the Open Test sequence eXchange (OTX) extension requirements and data model specifications. The requirements are derived from the use cases described in ISO 13209-1. They are listed in Clause 4. The data model specification aims at an exhaustive definition of all features of the OTX extensions which have been implemented to satisfy the requirements. This part of ISO 13209 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.