While keys cut across hierarchy
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Business rules
Keys let developers test relations between different parts of documents, and can be used to check things like "if X appears here, Y must appear over here." Keys are commonly used to ensure uniqueness within particular categories as well.
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XPath-based
Keys use a simple subset of XPath to define the relations between the pieces.
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Not particularly integrated
Keys and keyrefs aren't bonded tightly to the type structure, and rely on element and attribute names rather than their types.
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