The implementation of this element by reference means that the xlink:href element are pointing to a metadatarecord that contains a MD_DataIdentification object. The reference to the MD_DataIdentification object is done using the xpointer reference using the #-sign. The example above points to #lakes which is the ID of this object. Check example on xml encoding in section 0. The Unique Resource Identifier for the can be explicilty defined for the target dataset using the optional uuidref.attribute.