Section 4 defines MetadataSection@Identifier as: /mex:Metadata/mex:MetadataSection/@Identifier This indicates the Identifier for the metadata unit in this MetadataSection. Identifier is an absolute IRI. Back in issue http://www.w3.org/Bugs/Public/show_bug.cgi?id=12093 we agreed to make @Identifier required. Technically the spec is fine 'as is', but we really should modify the above to say: "This REQUIRED attribute indicates...." Basically just add "REQUIRED attribute" to make it clear that this attribute is not optional. The pseudo-xsd and real xsd already show it as required so this is more like a typo - or incomplete editor's work from 12093.