EXPRESS is a data modeling language designed to support the STEP data modeling activities within ISO.
EXPRESS-X is an extension to the EXPRESS language designed to support the translation of data populations from one EXPRESS schema to another.
All STEP documents are identified as ISO 10303-nnn, where nnn is the part number. The EXPRESS LRM (Language Reference Manual) is ISO 10303-11. The EXPRESS-X LRM is ISO 10303-14.
For more information about ISO, STEP, and EXPRESS follow these links: