XML Schema Complete Reference, The

6.16. Schema Element IDs

In an effort to demonstrate all the features offered by the XML Schema Recommendation, many of the examples in this book specify a unique ID for each schema component. Specifically, the elements have an id attribute. Each ID must be unique across the entire schema, not just within an XML schema document. Unless you are using the IDs for something meaningful, such as fragment identifiers, you might prefer not to specify IDs. The IDs tend to clutter the XML schema document.