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Schematron is an XML validation language which makes heavy use of xpath. It is used (along with XML schema) to constrain PDS4 labels and ensure that they are valid, according to a given stack of dictionaries referenced at the top of each label. It works by making assertions about the value or content of an element. If this assertion fails, a message is displayed to the user.

Some useful resources include:

In general, for PDS4 Schematron files (.sch) are not written directly, but are generated as the output of the LDDTool, which accepts as input an XML input file defining the dictionary attribute, classes and rules. In this case xpath is used in two contexts: