ISO/TS 10303-1275:2008 specifies the application module for External class.

The following are within the scope of ISO/TS 10303-1275:2008:

• the identification of a class that is external to a data exchange file;
• the identification of an external class library that is not in an exchange file and holds the definition for classes used in the exchange file;
• a reference to a class in an external class library;
• items within the scope of application module Class, ISO/TS 10303-1070;
• items within the scope of application module External source, ISO/TS 10303-1331;
• items within the scope of application module Name assignment, ISO/TS 10303-1340.