ISO/TS 10303-1780:2018-11 specifies the application module for
External unit.

The following are within the scope of
ISO/TS 10303-1780:2018-11:

• definition of a unit that is not included in the data exchange file and is defined in an external dictionary;
• items within the scope of application module External library, ISO/TS 10303-1778;
• items within the scope of application module Value with unit, ISO/TS 10303-1054.