"Are the semantics encoded in the XML Schema or in the structure of the XML instance documents that validate against that schema? Is it possible to 'understand' an instance document without reference to the schema?"
Possible answers are:
1. Yes: you can understand an XML instance document in the absence of a schema it validates against i.e. just from the structure of the elements and the namespaces used. 2. No: you require the XML Schema to understand the document.
Roger, If you postulate that the instance document is valid against the schema, and the that the element and attribute names are meaningful to the reader (a human, or software written by a human who understands their meaning), then the only additional semantics the schema could provide would be in the annotations/documentation, if any exist in the schema.
I'm not entirely sure what you include in [data] "structure", but if you only mean concepts such as tuples, trees, sets, lists, bags, etc., then I would disagree that semantics are encoded substantially in data structure (of the XML instance doc or any other record). It is true that without proper structure, semantics cannot be encoded, but I think semantics are encoded predominantly in class/element-attribute names and any referenced documentation (i.e., natural language). If you replace meaningful names with surrogate keys (e.g., integers) and thereby obscure any meaning conveyed by the names, then the instance document would lose a lot of its meaning.
I'm not exactly sure how this relates to the earlier discussion about XML schema, RDF, and more powerful modeling methodologies like UML. but I hope it helps.
Cheers,
-Stan