# XML Schema 1.0

The schema-aware version of the Saxon product (Saxon-SA) includes a complete implementation of XML Schema 1.0. This version of the product is available from Saxonica Limited.

Test results have been submitted against the W3C XML Schema Test Suite. All deviations from the expected test results have been accounted for, and are believed to represent areas where the tests are either incorrect, or where implementations can legitimately differ.

Known limitations include the following:

- Saxon imposes limits on the values of `minOccurs` and `maxOccurs` appearing on any particle other than an element particle in a content model. These limits are designed to prevent out-of-memory errors. By default, the upper limit for `minOccurs` is 100 and the upper limit for `maxOccurs` (other than "unbounded") is 250. If these limits are exceeded, a warning is output, and the values specified are replaced with the highest allowed value (which is "unbounded" in the case of `maxOccurs`). The limits are configurable using the method `setOccurrenceLimits()` in the class `SchemaAwareConfiguration`. If they are set too high, however, out-of-memory or stack overflow errors will occur either during schema compilation or during instance validation. From Saxon 9.1, these limits no longer apply to the common case of element particles, which are now handled using counters.

- Saxon implements schema processing only to the extent required for XPath, XSLT, and XQuery processing and for assessment of document validity. This means that PSVI properties beyond those required for the XPath data model are not provided. It also means that validation errors are fatal, they do not result in a PSVI that indicates the validation outcome for individual nodes.

- Elements and attributes of type `xs:ENTITY` or `xs:ENTITIES` are not checked against the list of unparsed entities declared in the document. (This check is not required during XSLT or XQuery validation, but it is required by standalone schema validation.)

- Saxon uses a different algorithm to evaluate type subsumption from the one given in the specification. The Saxon algorithm (an implementation of the algorithm published by Henry Thompson and Richard Tobin of the University of Edinburgh) is more accurate than the one in the W3C specification in deciding whether one complex type subsumes another. It therefore permits some complex type restrictions that the cruder W3C algorithm disallows.

- Saxon also uses the Thomson and Tobin algorithm to evaluate the Unique Particle Attribution constraint. The effect of this is that no UPA violation is reported when a complex type contains two distinct element particles that are references to the same element declaration, since the element declaration can be identified unambiguously.

- Saxon does not allow a user-defined type to be derived directly from `xs:anySimpleType`. It seems that the specification does not disallow this, though the effect of doing it is very unclear.

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