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Technical description - Part 28
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Part 28 of the ISO 10303 standard (STEP) describes an XML based format for file exchange.
Whereas the first edition of ISO10303-28 specified the structure of XML exchange documents using Document Type Definitions (DTDs), Edition 2 of ISO10303-28 (as used in PLCS) specifies exchange structures using XML Schema, and specifies the mapping of the EXPRESS data model to an XML Schema data model. XML schemas are increasingly used to validate XML documents and are slowly taking over from the DTD approach in many areas of XML application.
ISO 10303-28 allows for the use of several different options in constructing an XML schema from an EXPRESS schema. PLCS has adopted the default options in all cases save for the use of identity constraints. Such constraints (enforced through the use of the key/keyref mechanism) proscribe the use of nesting and prescribe the use of identifiers and references to those identifiers on the XML elements corresponding to EXPRESS entity datatypes.
The examples below illustrate EXPRESS fragments together with the corresponding items in the equivalent Part 28 XSD file and samples of data that accord with the XSD.