ISO 10303-14:2005 specifies a language for specifying relationships between data that are governed by EXPRESS schemas, and for specifying alternate views of data that are governed by EXPRESS schemas. The language is called EXPRESS-X.

EXPRESS-X is a structural data mapping language. It consists of language elements that allow an unambiguous specification of a relationship between EXPRESS schemas.

The following are within the scope of ISO 10303-14:2005:

- mapping of data governed by one EXPRESS schema to data governed by another EXPRESS schema;

- mapping of data governed by one version of an EXPRESS schema to data governed by another version of that EXPRESS schema, where the two schemas have different names;

- specification of requirements for data translators for data sharing and data exchange applications;

- specification of alternate views of data defined by an EXPRESS schema;

- an alternate notation for application protocol mapping tables;

- bidirectional mappings where mathematically possible;

- specification of constraints that may be evaluated against data produced by mapping.