XML Pocket Reference, 3rd Edition

XML Schema Constraining Facets

The XML Schema Part 2: Datatypes specification defines both fundamental facets and constraining facets. Fundamental facets (equal, ordered, bounded, cardinality, and numeric) are built into the definitions of the predefined datatypes and aren’t available for you to use in creating your own schemas. Constraining facets, on the other hand, allow you to specify more exactly the values or kinds of values may be stored in your schema components. Applying these facets in xs:restriction elements allows you to create more precise types than those that come built into XML Schema itself. Many of these facets apply only to particular types—for instance, xs:fractionDigits wouldn’t make much sense applied to strings.

All facet elements may contain xs:annotation child elements and have attributes outside the XML Schema namespace. They also share the fixed and id attributes. When the fixed attribute is set to true, the given facet cannot be modified during later restriction. The id attribute is useful if you’re creating schemas automatically and need guaranteed unique identifiers on every component.

Also, some facets cannot be applied simultaneously with others. If you need to set both, you’ll need to use two separate xs:restriction elements.