The gml:identifier value of the referenced feature, as a fully qualifiedvalue. It gives an alternative to the gml:id in xlink:href for findingthe referenced feature, mainly to reference a feature by a globallyunique identifier and where naming conflicts occur with gml:id whenaccessing data in external files.
<element name="observationSystemRef" type="diggs:FeatureReferenceType"><annotation><appinfo source="urn:x-gml:targetElement">diggs:AbstractObservationSystem</appinfo><documentation>A reference to an observation system, such as a lithology log, a stratigraphy or a geo-unit model. Given by reference only.</documentation></annotation></element>