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The Applicability Language specification defines a standardized structure for forming complex logical expressions out of Well-formed Names (WFNs). These expressions, also known as applicability statements, are used to tag checklists, policies, guidance, and other documents with information about the product(s) to which the documents apply. For example, a security checklist for Mozilla Firefox 3.6 running on Microsoft Windows Vista could be tagged with a single applicability statement that ensures only systems with both Mozilla Firefox 3.6 and Microsoft Windows Vista will have the security checklist applied.
XML Schema Files: [what is a schema?]
CPE 2.3 Applicability Language (XSD 1.0)