Validation can be about checking the structure of XML documents. It can be about checking the content of each text node and attribute independently of each other (datatype checking). It can be about checking constraints on relationships between nodes. It can be about checking constraints between nodes and external information such as lookup tables or links. It can be about checking business rules. Taken liberally, it can be almost anything else, even spell checking.

All of these aspects are important for improving the level of quality of XML-based information systems. I recently heard two presentations about two independent projects in very different domains. Both came out with this alarming ratio: one out of ten real-world XML documents contains errors. With such a high proportion, validation is not only useful but indispensable! The word "alarming" is not overstating the case—imagine a banking system where 10% of the transactions contain errors. Calling validation important, therefore, is an understatement.